diff --git a/IoListTestingWindow.EmbeddedEngineeringHost.cs b/IoListTestingWindow.EmbeddedEngineeringHost.cs
index 5e434facc..38a163cfb 100644
--- a/IoListTestingWindow.EmbeddedEngineeringHost.cs
+++ b/IoListTestingWindow.EmbeddedEngineeringHost.cs
@@ -28,6 +28,21 @@ internal static void RegisterEmbeddedEngineeringFatHost()
             handledEventsToo: true);
     }
 
+    internal void PrepareForEmbeddedEngineeringHost()
+    {
+        // WPF refuses Window.Show() when ShowActivated=false while WindowState=Maximized.
+        // The production FAT XAML historically starts maximized, therefore normalize the
+        // invisible donor BEFORE Show(). Its exact center is re-parented into MainWindow on
+        // Loaded; the donor itself never needs a maximized native HWND.
+        WindowState = WindowState.Normal;
+        ShowActivated = false;
+        ShowInTaskbar = false;
+        Opacity = 0d;
+        WindowStartupLocation = WindowStartupLocation.Manual;
+        Left = -32000d;
+        Top = -32000d;
+    }
+
     private static void EmbeddedEngineeringFatHost_Loaded(object sender, RoutedEventArgs e)
     {
         if (sender is not IoListTestingWindow window ||
@@ -45,9 +60,7 @@ window.Owner is not MainWindow owner ||
         // MainWindow's legacy launcher still calls Show() on this Window. Make that bootstrap
         // surface invisible immediately; the actual production visual is moved into Engineering
         // on the next Loaded-priority dispatcher turn, before ContextIdle command-panel work.
-        window.ShowActivated = false;
-        window.ShowInTaskbar = false;
-        window.Opacity = 0d;
+        window.PrepareForEmbeddedEngineeringHost();
 
         window.Dispatcher.BeginInvoke(
             DispatcherPriority.Loaded,
@@ -64,14 +77,13 @@ private void TryMountIntoEngineering(MainWindow owner)
         {
             EnsureProductionFatPresentationForEmbeddedHost();
             DisableLegacyEmbeddedCommandPanel();
-            var surface = DetachProductionFatCentralWorkspace();
-            if (surface == null)
-                return;
+            var surface = DetachProductionFatCentralWorkspace()
+                          ?? throw new InvalidOperationException("Production FAT center could not be detached from its donor window.");
 
             _engineeringEmbeddedSurface = surface;
             _engineeringEmbeddedMounted = owner.MountProductionFatWorkspace(this, surface);
             if (!_engineeringEmbeddedMounted)
-                return;
+                throw new InvalidOperationException("Engineering FAT host rejected the production workspace surface.");
 
             // The central FAT view now belongs to MainWindow. Keep this Window loaded and
             // hidden because existing controller/session/event code is intentionally reused.
@@ -79,9 +91,16 @@ private void TryMountIntoEngineering(MainWindow owner)
         }
         catch (Exception ex)
         {
+            // Never leave an invisible off-screen donor as a silent blank FAT failure.
+            // Restore the historical standalone presentation so the operator has a usable
+            // production FAT surface and a visible diagnostic if embedding itself fails.
+            WindowState = WindowState.Maximized;
             Opacity = 1d;
             ShowInTaskbar = true;
             ShowActivated = true;
+            WindowStartupLocation = WindowStartupLocation.CenterOwner;
+            Left = double.NaN;
+            Top = double.NaN;
             MessageBox.Show(
                 owner,
                 $"ARSAS could not embed the production FAT workspace. The standalone FAT window will remain available.\n\n{ex.Message}",
@@ -190,22 +209,51 @@ private void DisableLegacyEmbeddedCommandPanel()
 
     internal void SelectEngineeringDeviceForEmbeddedFat(Iec61850MonitorDevice? device)
     {
-        if (!_engineeringEmbeddedMounted || device == null)
+        if (!_engineeringEmbeddedMounted)
             return;
 
-        var match = Project.Ieds.FirstOrDefault(ied =>
-            (!string.IsNullOrWhiteSpace(ied.LiveDeviceId) &&
-             ied.LiveDeviceId.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase)) ||
-            (!string.IsNullOrWhiteSpace(ied.IpAddress) &&
-             ied.IpAddress.Equals(device.IpAddress, StringComparison.OrdinalIgnoreCase)) ||
-            ied.IedName.Equals(device.SclIedName, StringComparison.OrdinalIgnoreCase) ||
-            ied.IedName.Equals(device.Name, StringComparison.OrdinalIgnoreCase));
-        if (match == null || ReferenceEquals(SelectedIed, match))
+        // M3 viewed-device contract: the persistent Engineering IED Explorer owns
+        // what the FAT grid displays. SelectedIed/Session.SelectContext changes only that
+        // projection; an active capture remains latched inside its per-IED controller.
+        // Clearing the Explorer selection or selecting an IED outside this FAT projection
+        // must clear the FAT view instead of silently leaving the previous IED on screen.
+        if (device == null)
+        {
+            if (CanSelectIed)
+                SelectedIed = null;
             return;
+        }
+
+        // Resolve by strongest Engineering identity first. Do not use one OR predicate:
+        // a weak fallback on an earlier project row must never beat an exact live DeviceId.
+        var match = Project.Ieds.FirstOrDefault(_ => false);
+        if (!string.IsNullOrWhiteSpace(device.DeviceId))
+        {
+            match = Project.Ieds.FirstOrDefault(ied =>
+                !string.IsNullOrWhiteSpace(ied.LiveDeviceId) &&
+                ied.LiveDeviceId.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase));
+        }
+
+        if (match == null && !string.IsNullOrWhiteSpace(device.SclIedName))
+        {
+            match = Project.Ieds.FirstOrDefault(ied =>
+                ied.IedName.Equals(device.SclIedName, StringComparison.OrdinalIgnoreCase));
+        }
+
+        if (match == null && !string.IsNullOrWhiteSpace(device.Name))
+        {
+            match = Project.Ieds.FirstOrDefault(ied =>
+                ied.IedName.Equals(device.Name, StringComparison.OrdinalIgnoreCase));
+        }
+
+        if (match == null && !string.IsNullOrWhiteSpace(device.IpAddress))
+        {
+            match = Project.Ieds.FirstOrDefault(ied =>
+                !string.IsNullOrWhiteSpace(ied.IpAddress) &&
+                ied.IpAddress.Equals(device.IpAddress, StringComparison.OrdinalIgnoreCase));
+        }
 
-        // Do not retarget an active production FAT transaction/session. When idle, the
-        // persistent Engineering IED Explorer is the navigation/selection authority.
-        if (!CanSelectIed)
+        if (ReferenceEquals(SelectedIed, match) || !CanSelectIed)
             return;
 
         SelectedIed = match;
diff --git a/IoListTestingWindow.xaml b/IoListTestingWindow.xaml
index 48b1a7bb5..1b0803998 100644
--- a/IoListTestingWindow.xaml
+++ b/IoListTestingWindow.xaml
@@ -7,7 +7,7 @@
         Height="900" Width="1500" MinHeight="760" MinWidth="1180"
         WindowStartupLocation="CenterOwner" WindowState="Maximized"
         Background="{StaticResource AppBackgroundGradient}"
-        FontFamily="Aptos, Segoe UI Variable Text, Segoe UI, Calibri"
+        FontFamily="Inter, Segoe UI Variable Text, Segoe UI, Calibri"
         Icon="Assets/app-icon.ico"
         Closing="Window_Closing"
         ContentRendered="IoListTestingWindow_ContentRendered">
@@ -17,11 +17,11 @@
         <Style x:Key="FatGridText" TargetType="TextBlock">
             <Setter Property="VerticalAlignment" Value="Center"/>
             <Setter Property="Foreground" Value="{StaticResource Ink}"/>
-            <Setter Property="FontSize" Value="12.2"/>
+            <Setter Property="FontSize" Value="11.5"/>
             <Setter Property="TextTrimming" Value="CharacterEllipsis"/>
         </Style>
         <Style TargetType="DataGridCell">
-            <Setter Property="Padding" Value="8,6"/>
+            <Setter Property="Padding" Value="7,4"/>
             <Setter Property="BorderThickness" Value="0,0,0,1"/>
             <Setter Property="BorderBrush" Value="#E8EDF5"/>
             <Setter Property="Background" Value="Transparent"/>
@@ -29,11 +29,11 @@
             <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
         </Style>
         <Style TargetType="DataGridColumnHeader">
-            <Setter Property="Height" Value="38"/>
-            <Setter Property="Padding" Value="8,0"/>
+            <Setter Property="Height" Value="34"/>
+            <Setter Property="Padding" Value="7,0"/>
             <Setter Property="Background" Value="#F4F7FB"/>
             <Setter Property="Foreground" Value="#617087"/>
-            <Setter Property="FontSize" Value="10.5"/>
+            <Setter Property="FontSize" Value="10.2"/>
             <Setter Property="FontWeight" Value="SemiBold"/>
             <Setter Property="BorderThickness" Value="0,0,0,1"/>
             <Setter Property="BorderBrush" Value="#DCE4EF"/>
@@ -45,7 +45,7 @@
             <Setter Property="HorizontalContentAlignment" Value="Center"/>
         </Style>
         <Style TargetType="DataGridRow">
-            <Setter Property="MinHeight" Value="48"/>
+            <Setter Property="MinHeight" Value="40"/>
             <Setter Property="Background" Value="#FFFFFF"/>
             <Setter Property="BorderThickness" Value="0"/>
             <Style.Triggers>
@@ -59,16 +59,16 @@
         </Style>
     </Window.Resources>
 
-    <Grid Margin="14">
+    <Grid Margin="12">
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto"/>
-            <RowDefinition Height="12"/>
-            <RowDefinition Height="*"/>
             <RowDefinition Height="10"/>
+            <RowDefinition Height="*"/>
+            <RowDefinition Height="8"/>
             <RowDefinition Height="Auto"/>
         </Grid.RowDefinitions>
 
-        <Border Grid.Row="0" Style="{StaticResource Card}" Padding="16,12">
+        <Border Grid.Row="0" Style="{StaticResource Card}" Padding="14,10">
             <Grid>
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="*"/>
@@ -76,24 +76,24 @@
                 </Grid.ColumnDefinitions>
                 <StackPanel VerticalAlignment="Center">
                     <TextBlock Text="IO LIST FAT" Style="{StaticResource MicroLabel}" Foreground="{StaticResource Accent}"/>
-                    <TextBlock Text="{Binding Project.ProjectName}" FontSize="18" FontWeight="SemiBold"
+                    <TextBlock Text="{Binding Project.ProjectName}" FontSize="17" FontWeight="SemiBold"
                                Foreground="{StaticResource Ink}" Margin="0,2,0,0"/>
-                    <TextBlock Text="Workbook scope · report-first acquisition · relay-timestamped evidence"
+                    <TextBlock Text="Engineering SCL scope · report-first acquisition · relay-timestamped evidence"
                                Style="{StaticResource Caption}" Margin="0,3,0,0"/>
                 </StackPanel>
                 <WrapPanel Grid.Column="1" VerticalAlignment="Center" HorizontalAlignment="Right">
-                    <Border Background="#F1F5FB" CornerRadius="14" Padding="12,7" Margin="0,0,9,0">
-                        <TextBlock Text="{Binding ProjectSummary}" FontSize="11.8" Foreground="{StaticResource Ink}"/>
+                    <Border Background="#F1F5FB" CornerRadius="14" Padding="11,6" Margin="0,0,8,0">
+                        <TextBlock Text="{Binding ProjectSummary}" FontSize="11.4" Foreground="{StaticResource Ink}"/>
                     </Border>
-                    <Button Content="Save" Style="{StaticResource SoftButton}" Padding="11,7"
-                            Click="SaveProgress_Click" Margin="0,0,6,0"/>
-                    <Button Content="Excel" Style="{StaticResource SoftButton}" Padding="11,7"
-                            Click="ExportExcel_Click" Margin="0,0,6,0"/>
-                    <Button Content="PDF" Style="{StaticResource SoftButton}" Padding="11,7"
-                            Click="ExportPdf_Click" Margin="0,0,6,0"/>
-                    <Button Content="Export .arsas" Style="{StaticResource PrimaryButton}" Padding="12,7"
-                            Click="ExportHandover_Click" Margin="0,0,6,0"/>
-                    <Button Content="Engineering" Style="{StaticResource SoftButton}" Padding="11,7"
+                    <Button Content="Save" Style="{StaticResource SoftButton}" Padding="10,6"
+                            Click="SaveProgress_Click" Margin="0,0,5,0"/>
+                    <Button Content="Excel" Style="{StaticResource SoftButton}" Padding="10,6"
+                            Click="ExportExcel_Click" Margin="0,0,5,0"/>
+                    <Button Content="PDF" Style="{StaticResource SoftButton}" Padding="10,6"
+                            Click="ExportPdf_Click" Margin="0,0,5,0"/>
+                    <Button Content="Export .arsas" Style="{StaticResource PrimaryButton}" Padding="11,6"
+                            Click="ExportHandover_Click" Margin="0,0,5,0"/>
+                    <Button Content="Engineering" Style="{StaticResource SoftButton}" Padding="10,6"
                             Click="ReturnToEngineering_Click"/>
                 </WrapPanel>
             </Grid>
@@ -101,21 +101,21 @@
 
         <Grid Grid.Row="2">
             <Grid.ColumnDefinitions>
-                <ColumnDefinition Width="304"/>
-                <ColumnDefinition Width="12"/>
+                <ColumnDefinition Width="282"/>
+                <ColumnDefinition Width="10"/>
                 <ColumnDefinition Width="*"/>
             </Grid.ColumnDefinitions>
 
-            <Border Grid.Column="0" Style="{StaticResource Card}" Padding="10">
+            <Border Grid.Column="0" Style="{StaticResource Card}" Padding="9">
                 <DockPanel>
-                    <Grid DockPanel.Dock="Top" Margin="5,3,5,11">
+                    <Grid DockPanel.Dock="Top" Margin="5,3,5,9">
                         <Grid.ColumnDefinitions>
                             <ColumnDefinition Width="*"/>
                             <ColumnDefinition Width="Auto"/>
                         </Grid.ColumnDefinitions>
                         <StackPanel>
                             <TextBlock Text="IED EXPLORER" Style="{StaticResource MicroLabel}"/>
-                            <TextBlock Text="Workbook devices" Style="{StaticResource Caption}" Margin="0,2,0,0"/>
+                            <TextBlock Text="Engineering FAT devices" Style="{StaticResource Caption}" Margin="0,2,0,0"/>
                         </StackPanel>
                         <Border Grid.Column="1" Background="#F1F5FB" CornerRadius="12" Padding="8,4" VerticalAlignment="Center">
                             <TextBlock Text="{Binding Project.Ieds.Count}" FontWeight="SemiBold" FontSize="11"
@@ -124,7 +124,7 @@
                     </Grid>
 
                     <Button DockPanel.Dock="Bottom" Content="＋ Add IED · Import SCL"
-                            Style="{StaticResource PrimaryButton}" Margin="4,10,4,2" Padding="12,9"
+                            Style="{StaticResource PrimaryButton}" Margin="4,8,4,2" Padding="11,8"
                             IsEnabled="{Binding CanAddFatIed}" Click="AddFatIedFromScl_Click"
                             ToolTip="Add another IED from CID/SCD/ICD/IID without replacing the current FAT workspace"/>
 
@@ -137,7 +137,7 @@
                         <ListBox.ItemContainerStyle>
                             <Style TargetType="ListBoxItem">
                                 <Setter Property="Padding" Value="0"/>
-                                <Setter Property="Margin" Value="0,0,0,8"/>
+                                <Setter Property="Margin" Value="0,0,0,7"/>
                                 <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
                                 <Setter Property="Background" Value="Transparent"/>
                                 <Setter Property="BorderThickness" Value="0"/>
@@ -146,7 +146,7 @@
                                     <Setter.Value>
                                         <ControlTemplate TargetType="ListBoxItem">
                                             <Border x:Name="IedCard" Background="#FAFCFF" BorderBrush="#D8E1EE"
-                                                    BorderThickness="1" CornerRadius="15" Padding="10">
+                                                    BorderThickness="1" CornerRadius="14" Padding="9">
                                                 <ContentPresenter/>
                                             </Border>
                                             <ControlTemplate.Triggers>
@@ -166,14 +166,14 @@
                         </ListBox.ItemContainerStyle>
                         <ListBox.ItemTemplate>
                             <DataTemplate>
-                                <Grid MinHeight="70">
+                                <Grid MinHeight="64">
                                     <Grid.ColumnDefinitions>
-                                        <ColumnDefinition Width="58"/>
+                                        <ColumnDefinition Width="54"/>
                                         <ColumnDefinition Width="*"/>
                                     </Grid.ColumnDefinitions>
-                                    <StackPanel Grid.Column="0" Width="56"
+                                    <StackPanel Grid.Column="0" Width="52"
                                                 HorizontalAlignment="Center" VerticalAlignment="Center">
-                                        <Viewbox Width="46" Height="46" HorizontalAlignment="Center">
+                                        <Viewbox Width="42" Height="42" HorizontalAlignment="Center">
                                             <Control x:Name="RelayIcon" Width="50" Height="50" Foreground="#D94B55"
                                                      Template="{StaticResource IedRelayFrontPanelTemplate}"/>
                                         </Viewbox>
@@ -202,12 +202,12 @@
                                                 <ColumnDefinition Width="*"/>
                                                 <ColumnDefinition Width="Auto"/>
                                             </Grid.ColumnDefinitions>
-                                            <TextBlock Text="{Binding IedName}" FontSize="12.8" FontWeight="SemiBold"
+                                            <TextBlock Text="{Binding IedName}" FontSize="12.4" FontWeight="SemiBold"
                                                        Foreground="{StaticResource Ink}" TextTrimming="CharacterEllipsis"
-                                                       Margin="0,0,10,0" VerticalAlignment="Center"/>
+                                                       Margin="0,0,9,0" VerticalAlignment="Center"/>
                                             <Border Grid.Column="1" Background="#EAF8F1" BorderBrush="#8FD1B1"
                                                     BorderThickness="1" CornerRadius="9" Padding="7,2"
-                                                    Margin="10,0,0,0" VerticalAlignment="Center">
+                                                    Margin="9,0,0,0" VerticalAlignment="Center">
                                                 <Border.Visibility>
                                                     <MultiBinding Converter="{StaticResource AllPassedVisibilityConverter}">
                                                         <Binding Path="TestPoints.Count"/>
@@ -218,8 +218,8 @@
                                             </Border>
                                         </Grid>
                                         <TextBlock Grid.Row="1" Text="{Binding IpAddress}" Style="{StaticResource Caption}"
-                                                   Margin="0,3,0,0"/>
-                                        <TextBlock Grid.Row="2" Margin="0,7,0,0" FontSize="10.5"
+                                                   Margin="0,2,0,0"/>
+                                        <TextBlock Grid.Row="2" Margin="0,6,0,0" FontSize="10.2"
                                                    Foreground="{StaticResource Muted}">
                                             <Run Text="{Binding BoundCount, Mode=OneWay}"/>
                                             <Run Text=" live  ·  "/>
@@ -231,7 +231,7 @@
                                         <ProgressBar x:Name="CardProgress"
                                                      Grid.Row="3"
                                                      Height="3"
-                                                     Margin="0,8,0,0"
+                                                     Margin="0,7,0,0"
                                                      Minimum="0"
                                                      Maximum="100"
                                                      IsIndeterminate="True"
@@ -268,12 +268,12 @@
                 </DockPanel>
             </Border>
 
-            <Border Grid.Column="2" Style="{StaticResource Card}" Padding="12">
+            <Border Grid.Column="2" Style="{StaticResource Card}" Padding="10">
                 <Grid>
                     <Grid.RowDefinitions>
                         <RowDefinition Height="Auto"/>
                         <RowDefinition Height="Auto"/>
-                        <RowDefinition Height="12"/>
+                        <RowDefinition Height="10"/>
                         <RowDefinition Height="*"/>
                     </Grid.RowDefinitions>
 
@@ -282,47 +282,47 @@
                             <ColumnDefinition Width="*"/>
                             <ColumnDefinition Width="Auto"/>
                         </Grid.ColumnDefinitions>
-                        <StackPanel VerticalAlignment="Center" Margin="0,0,14,0">
-                            <TextBlock Text="{Binding SelectedIed.IedName}" FontSize="17" FontWeight="SemiBold"
+                        <StackPanel VerticalAlignment="Center" Margin="0,0,12,0">
+                            <TextBlock Text="{Binding SelectedIed.IedName}" FontSize="16" FontWeight="SemiBold"
                                        Foreground="{StaticResource Ink}"/>
-                            <TextBlock Text="{Binding SelectedIedSummary}" Style="{StaticResource Caption}" Margin="0,4,0,0"/>
+                            <TextBlock Text="{Binding SelectedIedSummary}" Style="{StaticResource Caption}" Margin="0,3,0,0"/>
                         </StackPanel>
                         <StackPanel Grid.Column="1" Orientation="Horizontal" VerticalAlignment="Center">
                             <Button x:Name="WorkspacePreviewToggle"
                                     Content="Print Preview"
                                     Style="{StaticResource SoftButton}"
-                                    Padding="11,8"
+                                    Padding="10,7"
                                     Click="TogglePrintPreview_Click"
-                                    Margin="0,0,10,0"/>
+                                    Margin="0,0,8,0"/>
                             <Button Content="{Binding SelectedStartWorkflowText}" Style="{StaticResource PrimaryButton}"
-                                    Padding="13,8" IsEnabled="{Binding SelectedCanStartWorkflow}"
-                                    Click="StartSelectedIedSafely_Click" Margin="0,0,6,0"/>
-                            <Button Content="Pause" Style="{StaticResource SoftButton}" Padding="11,8"
-                                    IsEnabled="{Binding SelectedCanPause}" Click="PauseSession_Click" Margin="0,0,6,0"/>
-                            <Button Content="Resume" Style="{StaticResource SoftButton}" Padding="11,8"
-                                    IsEnabled="{Binding SelectedCanResume}" Click="ResumeSession_Click" Margin="0,0,6,0"/>
-                            <Button Content="Stop" Style="{StaticResource SoftButton}" Padding="11,8"
+                                    Padding="12,7" IsEnabled="{Binding SelectedCanStartWorkflow}"
+                                    Click="StartSelectedIedSafely_Click" Margin="0,0,5,0"/>
+                            <Button Content="Pause" Style="{StaticResource SoftButton}" Padding="10,7"
+                                    IsEnabled="{Binding SelectedCanPause}" Click="PauseSession_Click" Margin="0,0,5,0"/>
+                            <Button Content="Resume" Style="{StaticResource SoftButton}" Padding="10,7"
+                                    IsEnabled="{Binding SelectedCanResume}" Click="ResumeSession_Click" Margin="0,0,5,0"/>
+                            <Button Content="Stop" Style="{StaticResource SoftButton}" Padding="10,7"
                                     IsEnabled="{Binding SelectedCanStop}" Click="StopSession_Click"/>
                         </StackPanel>
                     </Grid>
 
                     <Border Grid.Row="1" Visibility="{Binding PreparationVisibility}"
                             Background="#F3F7FF" BorderBrush="#B9CCF4" BorderThickness="1"
-                            CornerRadius="13" Padding="12,9" Margin="0,11,0,0">
+                            CornerRadius="12" Padding="11,8" Margin="0,9,0,0">
                         <StackPanel>
                             <Grid>
                                 <Grid.ColumnDefinitions>
                                     <ColumnDefinition Width="Auto"/>
                                     <ColumnDefinition Width="*"/>
                                 </Grid.ColumnDefinitions>
-                                <TextBlock Text="{Binding PreparationIedText}" FontSize="11.8" FontWeight="SemiBold"
+                                <TextBlock Text="{Binding PreparationIedText}" FontSize="11.5" FontWeight="SemiBold"
                                            Foreground="{StaticResource Ink}" VerticalAlignment="Center"/>
-                                <TextBlock Grid.Column="1" Text="{Binding PreparationStatusText}" FontSize="10.8"
-                                           Foreground="#526783" Margin="12,0,0,0" VerticalAlignment="Center"
+                                <TextBlock Grid.Column="1" Text="{Binding PreparationStatusText}" FontSize="10.6"
+                                           Foreground="#526783" Margin="11,0,0,0" VerticalAlignment="Center"
                                            TextTrimming="CharacterEllipsis"/>
                             </Grid>
                             <ProgressBar Height="4"
-                                         Margin="0,9,0,0"
+                                         Margin="0,8,0,0"
                                          IsIndeterminate="True"
                                          BorderThickness="0"
                                          Background="#DCE7F8"
@@ -349,16 +349,16 @@
                               SelectionMode="Single"
                               SelectionUnit="FullRow"
                               RowHeaderWidth="0"
-                               FrozenColumnCount="1"
-                               EnableRowVirtualization="True"
-                               EnableColumnVirtualization="True"
-                               VirtualizingPanel.IsVirtualizing="True"
-                               VirtualizingPanel.VirtualizationMode="Recycling"
-                               ScrollViewer.CanContentScroll="True"
-                               ScrollViewer.HorizontalScrollBarVisibility="Auto"
+                              FrozenColumnCount="1"
+                              EnableRowVirtualization="True"
+                              EnableColumnVirtualization="True"
+                              VirtualizingPanel.IsVirtualizing="True"
+                              VirtualizingPanel.VirtualizationMode="Recycling"
+                              ScrollViewer.CanContentScroll="True"
+                              ScrollViewer.HorizontalScrollBarVisibility="Auto"
                               ScrollViewer.VerticalScrollBarVisibility="Auto">
                         <DataGrid.Columns>
-                            <DataGridTemplateColumn Header="TEST" Width="48" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="TEST" Width="46" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <CheckBox IsChecked="{Binding TestEnabled, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
@@ -367,7 +367,7 @@
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="SIGNAL" Width="1.55*" MinWidth="190" MaxWidth="300">
+                            <DataGridTemplateColumn Header="SIGNAL" Width="1.35*" MinWidth="170" MaxWidth="260">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding SignalName}" Style="{StaticResource FatGridText}"
@@ -375,20 +375,20 @@
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="IEC REFERENCE" Width="1.85*" MinWidth="235" MaxWidth="360">
+                            <DataGridTemplateColumn Header="IEC REFERENCE" Width="2.25*" MinWidth="290" MaxWidth="480">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding ObjectReference}" Style="{StaticResource FatGridText}"
-                                                   FontFamily="Cascadia Mono, Consolas" FontSize="11.0"
+                                                   FontFamily="Cascadia Mono, Consolas" FontSize="10.8"
                                                    Foreground="#52627A" ToolTip="{Binding ObjectReference}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="LIVE" Width="82" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="LIVE" Width="76" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock x:Name="LiveText" Text="{Binding LiveBindingText}"
-                                                   Style="{StaticResource FatGridText}" FontSize="10.8"
+                                                   Style="{StaticResource FatGridText}" FontSize="10.5"
                                                    FontWeight="SemiBold" Foreground="#65758B"
                                                    ToolTip="{Binding LiveBindingReason}"/>
                                         <DataTemplate.Triggers>
@@ -399,7 +399,7 @@
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="VALUE" Width="78" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="VALUE" Width="72" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.CurrentValue}" Style="{StaticResource FatGridText}"
@@ -407,60 +407,60 @@
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="QUALITY" Width="88" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="QUALITY" Width="82" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.CurrentQuality}" Style="{StaticResource FatGridText}"
-                                                   FontSize="11.2" ToolTip="{Binding Runtime.CurrentQuality}"/>
+                                                   FontSize="10.8" ToolTip="{Binding Runtime.CurrentQuality}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="ACQUISITION" Width="136" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="ACQUISITION" Width="122" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.CurrentSource}" Style="{StaticResource FatGridText}"
-                                                   FontSize="10.8" FontWeight="SemiBold" Foreground="#405B7D"
+                                                   FontSize="10.4" FontWeight="SemiBold" Foreground="#405B7D"
                                                    ToolTip="{Binding Runtime.CurrentSource}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="ON · RELAY TIME" Width="168"
+                            <DataGridTemplateColumn Header="ON · RELAY TIME" Width="154"
                                                     HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.OnRelayTimestampText}"
                                                    Style="{StaticResource FatGridText}"
-                                                   FontFamily="Cascadia Mono, Consolas" FontSize="10.4"
+                                                   FontFamily="Cascadia Mono, Consolas" FontSize="10.1"
                                                    ToolTip="{Binding Runtime.OnEvidenceToolTip}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="OFF · RELAY TIME" Width="168"
+                            <DataGridTemplateColumn Header="OFF · RELAY TIME" Width="154"
                                                     HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.OffRelayTimestampText}"
                                                    Style="{StaticResource FatGridText}"
-                                                   FontFamily="Cascadia Mono, Consolas" FontSize="10.4"
+                                                   FontFamily="Cascadia Mono, Consolas" FontSize="10.1"
                                                    ToolTip="{Binding Runtime.OffEvidenceToolTip}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="STATUS" Width="118" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="STATUS" Width="108" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock Text="{Binding Runtime.StateText}" Style="{StaticResource FatGridText}"
-                                                   FontSize="10.7" FontWeight="SemiBold" Foreground="#607089"
+                                                   FontSize="10.4" FontWeight="SemiBold" Foreground="#607089"
                                                    ToolTip="{Binding Runtime.StatusReason}"/>
                                     </DataTemplate>
                                 </DataGridTemplateColumn.CellTemplate>
                             </DataGridTemplateColumn>
-                            <DataGridTemplateColumn Header="RESULT" Width="92" HeaderStyle="{StaticResource CenteredFatGridHeader}">
+                            <DataGridTemplateColumn Header="RESULT" Width="86" HeaderStyle="{StaticResource CenteredFatGridHeader}">
                                 <DataGridTemplateColumn.CellTemplate>
                                     <DataTemplate>
                                         <TextBlock x:Name="FinalResultText" Text="—"
                                                    Style="{StaticResource FatGridText}"
-                                                   FontSize="10.8" FontWeight="Bold"
+                                                   FontSize="10.5" FontWeight="Bold"
                                                    Foreground="#8A97A9"
                                                    ToolTip="{Binding Runtime.StatusReason}"/>
                                         <DataTemplate.Triggers>
@@ -490,7 +490,7 @@
         </Grid>
 
         <Border Grid.Row="4" Background="#F8FAFD" BorderBrush="#DEE6F0"
-                BorderThickness="1" CornerRadius="13" Padding="12,8">
+                BorderThickness="1" CornerRadius="12" Padding="11,7">
             <Grid>
                 <Grid.ColumnDefinitions>
                     <ColumnDefinition Width="1.4*"/>
@@ -515,4 +515,4 @@
             </Grid>
         </Border>
     </Grid>
-</Window>
+</Window>
\ No newline at end of file
diff --git a/IoListTestingWindow.xaml.cs b/IoListTestingWindow.xaml.cs
index 84f37fb94..c1ec2bcea 100644
--- a/IoListTestingWindow.xaml.cs
+++ b/IoListTestingWindow.xaml.cs
@@ -119,18 +119,28 @@ private void SetAddingFatIeds(bool value)
 
     private async void StartSession_Click(object sender, RoutedEventArgs e)
     {
-        var selectedIed = SelectedIed;
-        if (selectedIed?.IsPreparing == true)
+        // M4 freezes both the IED owner and exact evidence scope before any asynchronous
+        // Engineering preparation. Explorer navigation may continue, but it cannot redirect
+        // this transaction or silently widen the production capture scope.
+        var requestedIed = SelectedIed;
+        if (requestedIed?.IsPreparing == true)
             return;
 
-        var preflight = IoTestSessionPreflight.Validate(selectedIed);
-        if (!preflight.Succeeded)
+        IoFatCaptureTargetLease captureLease;
+        try
+        {
+            captureLease = IoFatProductionControllerAdapter.LatchStartTarget(Project, requestedIed);
+        }
+        catch (InvalidOperationException ex)
         {
-            ShowActionResult(preflight, "FAT session scope is not ready");
+            ShowActionResult(
+                IoTestSessionActionResult.Failure(ex.Message),
+                "FAT session scope is not ready");
             return;
         }
 
-        PreparationStatusText = $"Connecting {selectedIed!.IedName} · {selectedIed.IpAddress}:102";
+        var selectedIed = captureLease.Ied;
+        PreparationStatusText = $"Connecting {selectedIed.IedName} · {selectedIed.IpAddress}:102";
         RaisePreparationProperties();
         try
         {
@@ -155,7 +165,10 @@ private async void StartSession_Click(object sender, RoutedEventArgs e)
                 }
             }
 
-            var result = Session.Start(selectedIed);
+            // Revalidate the frozen identity/configuration only after Engineering has
+            // completed connection preparation. The adapter delegates to the unchanged
+            // production session controller, which remains the sole evidence writer.
+            var result = IoFatProductionControllerAdapter.StartLatched(Project, Session, captureLease);
             ShowActionResult(result, "FAT evidence session could not start");
             RaiseStatusProperties();
             if (result.Succeeded)
@@ -187,7 +200,8 @@ private async void StartSession_Click(object sender, RoutedEventArgs e)
 
     private void PauseSession_Click(object sender, RoutedEventArgs e)
     {
-        var result = Session.Pause();
+        var targetIed = SelectedIed;
+        var result = Session.Pause(targetIed);
         ShowActionResult(result, "FAT session could not pause");
         if (result.Succeeded)
             Storage?.SaveNow();
@@ -195,7 +209,10 @@ private void PauseSession_Click(object sender, RoutedEventArgs e)
 
     private void ResumeSession_Click(object sender, RoutedEventArgs e)
     {
-        var result = Session.Resume();
+        // Continue is explicitly IED-targeted. A later Explorer change cannot make the
+        // production controller rebind this active evidence session to another device.
+        var targetIed = SelectedIed;
+        var result = Session.Resume(targetIed);
         ShowActionResult(result, "FAT session could not resume");
         if (result.Succeeded)
             Storage?.ScheduleSave();
@@ -203,7 +220,8 @@ private void ResumeSession_Click(object sender, RoutedEventArgs e)
 
     private void StopSession_Click(object sender, RoutedEventArgs e)
     {
-        var result = Session.Stop();
+        var targetIed = SelectedIed;
+        var result = Session.Stop(targetIed);
         ShowActionResult(result, "FAT session could not stop");
         if (result.Succeeded)
             Storage?.SaveNow();
diff --git a/MainWindow.IoTesting.cs b/MainWindow.IoTesting.cs
index 52f1cfc91..69acfa89f 100644
--- a/MainWindow.IoTesting.cs
+++ b/MainWindow.IoTesting.cs
@@ -494,6 +494,8 @@ private Task ShowIoTestingWorkspaceAsync(IoTestWorkspaceLaunchResult launch, int
         var controller = launch.Session;
         var persistence = launch.Workspace;
         var window = new IoListTestingWindow(launch.Project, controller, persistence) { Owner = this };
+        if (ProductionFatTabReady)
+            window.PrepareForEmbeddedEngineeringHost();
         RegisterLoadedIoFatWindow(window);
         _activeIoTestSessionController = controller;
         Interlocked.Exchange(ref _ioTestObservationSequence, DateTime.UtcNow.Ticks);
@@ -525,7 +527,8 @@ void WindowClosed(object? sender, EventArgs args)
         }
 
         window.Closed += WindowClosed;
-        Hide();
+        if (!ProductionFatTabReady)
+            Hide();
         window.Show();
         return Task.CompletedTask;
     }
diff --git a/MainWindow.NativeFatExplorerSync.cs b/MainWindow.NativeFatExplorerSync.cs
deleted file mode 100644
index 8ea74646d..000000000
--- a/MainWindow.NativeFatExplorerSync.cs
+++ /dev/null
@@ -1,213 +0,0 @@
-using System.Collections.Specialized;
-using System.ComponentModel;
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Threading;
-using ArIED61850Tester.Models;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// Keeps the native FAT projection aligned with the persistent IED Explorer's signal
-/// selection without turning FAT into a second signal database.
-///
-/// ObservableCollection changes were already reconciled by the first P2 slice. Explorer
-/// checkbox changes are different: SignalDefinition stays in the collection and only
-/// IsSelected changes. This bridge observes that property, marks the FAT scope dirty, and
-/// forces non-destructive reconciliation when FAT is visible (or the next time it opens).
-/// </summary>
-public partial class MainWindow
-{
-    private DispatcherTimer? _nativeFatExplorerSyncInstallRetry;
-    private DispatcherTimer? _nativeFatExplorerScopeTimer;
-    private Iec61850MonitorDevice? _nativeFatExplorerSyncDevice;
-    private readonly HashSet<SignalDefinition> _nativeFatExplorerObservedSignals = new();
-    private bool _nativeFatExplorerSyncAttached;
-    private bool _nativeFatExplorerScopeDirty;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatExplorerSelectionSync()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatExplorerSync_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatExplorerSync_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatExplorerSyncAttached)
-            return;
-
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryAttachNativeFatExplorerSelectionSync));
-    }
-
-    private void TryAttachNativeFatExplorerSelectionSync()
-    {
-        if (_nativeFatExplorerSyncAttached || !IsLoaded)
-            return;
-
-        if (!_nativeFatInstalled)
-        {
-            _nativeFatExplorerSyncInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
-            {
-                Interval = TimeSpan.FromMilliseconds(180)
-            };
-            _nativeFatExplorerSyncInstallRetry.Tick -= NativeFatExplorerSyncInstallRetry_Tick;
-            _nativeFatExplorerSyncInstallRetry.Tick += NativeFatExplorerSyncInstallRetry_Tick;
-            _nativeFatExplorerSyncInstallRetry.Start();
-            return;
-        }
-
-        _nativeFatExplorerSyncInstallRetry?.Stop();
-        _nativeFatExplorerSyncAttached = true;
-
-        _nativeFatExplorerScopeTimer = new DispatcherTimer(DispatcherPriority.Background)
-        {
-            Interval = TimeSpan.FromMilliseconds(180)
-        };
-        _nativeFatExplorerScopeTimer.Tick += NativeFatExplorerScopeTimer_Tick;
-
-        PropertyChanged += NativeFatExplorerSync_MainWindowPropertyChanged;
-        MainTabs.SelectionChanged += NativeFatExplorerSync_MainTabsSelectionChanged;
-        Closed += NativeFatExplorerSync_MainWindowClosed;
-        RebindNativeFatExplorerSignals(SelectedDevice);
-    }
-
-    private void NativeFatExplorerSyncInstallRetry_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatExplorerSyncInstallRetry?.Stop();
-        TryAttachNativeFatExplorerSelectionSync();
-    }
-
-    private void NativeFatExplorerSync_MainWindowPropertyChanged(object? sender, PropertyChangedEventArgs e)
-    {
-        if (e.PropertyName != nameof(SelectedDevice))
-            return;
-
-        RebindNativeFatExplorerSignals(SelectedDevice);
-        MarkNativeFatExplorerScopeDirty();
-    }
-
-    private void RebindNativeFatExplorerSignals(Iec61850MonitorDevice? device)
-    {
-        if (ReferenceEquals(_nativeFatExplorerSyncDevice, device))
-        {
-            SyncNativeFatExplorerSignalSubscriptions();
-            return;
-        }
-
-        if (_nativeFatExplorerSyncDevice != null)
-            _nativeFatExplorerSyncDevice.Signals.CollectionChanged -= NativeFatExplorerSignals_CollectionChanged;
-
-        foreach (var signal in _nativeFatExplorerObservedSignals)
-            signal.PropertyChanged -= NativeFatExplorerSignal_PropertyChanged;
-        _nativeFatExplorerObservedSignals.Clear();
-
-        _nativeFatExplorerSyncDevice = device;
-        if (_nativeFatExplorerSyncDevice != null)
-        {
-            _nativeFatExplorerSyncDevice.Signals.CollectionChanged += NativeFatExplorerSignals_CollectionChanged;
-            SyncNativeFatExplorerSignalSubscriptions();
-        }
-    }
-
-    private void NativeFatExplorerSignals_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
-    {
-        SyncNativeFatExplorerSignalSubscriptions();
-        MarkNativeFatExplorerScopeDirty();
-    }
-
-    private void SyncNativeFatExplorerSignalSubscriptions()
-    {
-        var current = _nativeFatExplorerSyncDevice?.Signals.ToHashSet() ?? new HashSet<SignalDefinition>();
-
-        foreach (var signal in _nativeFatExplorerObservedSignals.Where(signal => !current.Contains(signal)).ToArray())
-        {
-            signal.PropertyChanged -= NativeFatExplorerSignal_PropertyChanged;
-            _nativeFatExplorerObservedSignals.Remove(signal);
-        }
-
-        foreach (var signal in current)
-        {
-            if (!_nativeFatExplorerObservedSignals.Add(signal))
-                continue;
-            signal.PropertyChanged += NativeFatExplorerSignal_PropertyChanged;
-        }
-    }
-
-    private void NativeFatExplorerSignal_PropertyChanged(object? sender, PropertyChangedEventArgs e)
-    {
-        // IsSelected is the Explorer membership authority. Value/quality/timestamp changes
-        // stay live-bound directly and must never trigger a full FAT reconciliation.
-        if (e.PropertyName == nameof(SignalDefinition.IsSelected))
-            MarkNativeFatExplorerScopeDirty();
-    }
-
-    private void MarkNativeFatExplorerScopeDirty()
-    {
-        _nativeFatExplorerScopeDirty = true;
-        if (MainTabs.SelectedIndex != NativeFatWorkspaceIndex)
-            return;
-
-        _nativeFatExplorerScopeTimer?.Stop();
-        _nativeFatExplorerScopeTimer?.Start();
-    }
-
-    private async void NativeFatExplorerScopeTimer_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatExplorerScopeTimer?.Stop();
-        if (!_nativeFatExplorerScopeDirty || MainTabs.SelectedIndex != NativeFatWorkspaceIndex)
-            return;
-
-        await EnsureNativeFatLoadedAsync(forceReconcile: true);
-        _nativeFatExplorerScopeDirty = false;
-    }
-
-    private void NativeFatExplorerSync_MainTabsSelectionChanged(object sender, SelectionChangedEventArgs e)
-    {
-        if (!ReferenceEquals(e.Source, MainTabs) ||
-            MainTabs.SelectedIndex != NativeFatWorkspaceIndex ||
-            !_nativeFatExplorerScopeDirty)
-        {
-            return;
-        }
-
-        // The base P2 selection handler may have issued a fast non-forced load already.
-        // Re-run at ContextIdle with forceReconcile so checkbox changes made while another
-        // workspace was active are never hidden behind the current-state fast path.
-        Dispatcher.BeginInvoke(
-            DispatcherPriority.ContextIdle,
-            new Action(async () =>
-            {
-                if (MainTabs.SelectedIndex != NativeFatWorkspaceIndex || !_nativeFatExplorerScopeDirty)
-                    return;
-                await EnsureNativeFatLoadedAsync(forceReconcile: true);
-                _nativeFatExplorerScopeDirty = false;
-            }));
-    }
-
-    private void NativeFatExplorerSync_MainWindowClosed(object? sender, EventArgs e)
-    {
-        _nativeFatExplorerSyncInstallRetry?.Stop();
-        _nativeFatExplorerScopeTimer?.Stop();
-
-        PropertyChanged -= NativeFatExplorerSync_MainWindowPropertyChanged;
-        MainTabs.SelectionChanged -= NativeFatExplorerSync_MainTabsSelectionChanged;
-        Closed -= NativeFatExplorerSync_MainWindowClosed;
-
-        if (_nativeFatExplorerSyncDevice != null)
-            _nativeFatExplorerSyncDevice.Signals.CollectionChanged -= NativeFatExplorerSignals_CollectionChanged;
-        foreach (var signal in _nativeFatExplorerObservedSignals)
-            signal.PropertyChanged -= NativeFatExplorerSignal_PropertyChanged;
-
-        _nativeFatExplorerObservedSignals.Clear();
-        _nativeFatExplorerSyncDevice = null;
-        _nativeFatExplorerSyncAttached = false;
-        _nativeFatExplorerScopeDirty = false;
-    }
-}
diff --git a/MainWindow.NativeFatExport.cs b/MainWindow.NativeFatExport.cs
deleted file mode 100644
index 426cd18c8..000000000
--- a/MainWindow.NativeFatExport.cs
+++ /dev/null
@@ -1,166 +0,0 @@
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Threading;
-using ArIED61850Tester.Services;
-using Microsoft.Win32;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// Native evidence export entry point. Export consumes the immutable report snapshot
-/// created from the already-loaded FAT state; it never creates a second IED session.
-/// </summary>
-public partial class MainWindow
-{
-    private DispatcherTimer? _nativeFatExportInstallRetry;
-    private Button? _nativeFatExportPdfButton;
-    private bool _nativeFatExportInstalled;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatExport()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatExport_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatExport_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatExportInstalled)
-            return;
-
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryInstallNativeFatExport));
-    }
-
-    private void TryInstallNativeFatExport()
-    {
-        if (_nativeFatExportInstalled || !IsLoaded)
-            return;
-
-        if (!_nativeFatInstalled || _nativeFatSearchBox?.Parent is not WrapPanel toolbar)
-        {
-            _nativeFatExportInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
-            {
-                Interval = TimeSpan.FromMilliseconds(190)
-            };
-            _nativeFatExportInstallRetry.Tick -= NativeFatExportInstallRetry_Tick;
-            _nativeFatExportInstallRetry.Tick += NativeFatExportInstallRetry_Tick;
-            _nativeFatExportInstallRetry.Start();
-            return;
-        }
-
-        _nativeFatExportInstallRetry?.Stop();
-        _nativeFatExportInstalled = true;
-        _nativeFatExportPdfButton = CreateNativeFatButton(
-            "Export PDF",
-            NativeFatExportPdf_Click,
-            "Export an immutable native FAT evidence PDF for the selected IED. Show historical controls whether historical signal rows are included.");
-
-        var historyIndex = _nativeFatShowHistoricalCheck == null
-            ? toolbar.Children.Count
-            : toolbar.Children.IndexOf(_nativeFatShowHistoricalCheck);
-        if (historyIndex < 0)
-            historyIndex = toolbar.Children.Count;
-        toolbar.Children.Insert(historyIndex, _nativeFatExportPdfButton);
-
-        Closed += NativeFatExport_MainWindowClosed;
-    }
-
-    private void NativeFatExportInstallRetry_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatExportInstallRetry?.Stop();
-        TryInstallNativeFatExport();
-    }
-
-    private async void NativeFatExportPdf_Click(object sender, RoutedEventArgs e)
-    {
-        if (_nativeFatExportPdfButton == null)
-            return;
-
-        await EnsureNativeFatLoadedAsync(forceReconcile: false);
-        var device = SelectedDevice;
-        var state = _nativeFatCurrentState;
-        if (device == null || state == null ||
-            !state.DeviceId.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase))
-        {
-            SetNativeFatStatus("Select an IED with a loaded FAT state before exporting.");
-            return;
-        }
-
-        var includeHistorical = _nativeFatShowHistoricalCheck?.IsChecked == true;
-        var snapshot = NativeFatReportSnapshotBuilder.Build(device, state);
-        var dialog = new SaveFileDialog
-        {
-            Title = "Export ARSAS native FAT evidence PDF",
-            Filter = "PDF evidence report (*.pdf)|*.pdf",
-            FileName = $"{SafeNativeFatFileName(device.Name)}_FAT_{DateTime.Now:yyyyMMdd_HHmm}.pdf",
-            AddExtension = true,
-            DefaultExt = ".pdf",
-            OverwritePrompt = true
-        };
-        if (dialog.ShowDialog(this) != true)
-            return;
-
-        var previousContent = _nativeFatExportPdfButton.Content;
-        try
-        {
-            _nativeFatExportPdfButton.IsEnabled = false;
-            _nativeFatExportPdfButton.Content = "Exporting…";
-            SetNativeFatStatus("Saving current FAT state and building evidence PDF…");
-
-            // Persist the exact state used to construct the snapshot before delivering
-            // an external evidence file. The snapshot itself remains frozen thereafter.
-            await SaveNativeFatStateAsync(state);
-            await Task.Run(() => NativeFatPdfReportService.Save(
-                dialog.FileName,
-                snapshot,
-                includeHistorical));
-
-            SetNativeFatStatus($"PDF exported · {Path.GetFileName(dialog.FileName)}");
-            AddLog(
-                "INFO",
-                "Native FAT",
-                $"Native FAT PDF exported for {device.Name}: {dialog.FileName}" +
-                (includeHistorical ? " (historical included)" : string.Empty));
-        }
-        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException or InvalidDataException or InvalidOperationException or ArgumentException)
-        {
-            AddLog("WARN", "Native FAT", $"PDF export failed: {ex.Message}");
-            SetNativeFatStatus("PDF export failed. Native FAT state remains saved.");
-            MessageBox.Show(
-                this,
-                ex.Message,
-                "Native FAT PDF export failed",
-                MessageBoxButton.OK,
-                MessageBoxImage.Error);
-        }
-        finally
-        {
-            _nativeFatExportPdfButton.Content = previousContent ?? "Export PDF";
-            _nativeFatExportPdfButton.IsEnabled = true;
-        }
-    }
-
-    private static string SafeNativeFatFileName(string? value)
-    {
-        var source = string.IsNullOrWhiteSpace(value) ? "IED" : value.Trim();
-        var invalid = Path.GetInvalidFileNameChars().ToHashSet();
-        var result = new string(source.Select(character => invalid.Contains(character) ? '_' : character).ToArray())
-            .Trim()
-            .Trim('.');
-        return string.IsNullOrWhiteSpace(result) ? "IED" : result;
-    }
-
-    private void NativeFatExport_MainWindowClosed(object? sender, EventArgs e)
-    {
-        _nativeFatExportInstallRetry?.Stop();
-        if (_nativeFatExportPdfButton != null)
-            _nativeFatExportPdfButton.Click -= NativeFatExportPdf_Click;
-        Closed -= NativeFatExport_MainWindowClosed;
-    }
-}
diff --git a/MainWindow.NativeFatHistoryInspector.cs b/MainWindow.NativeFatHistoryInspector.cs
deleted file mode 100644
index 27b24d82c..000000000
--- a/MainWindow.NativeFatHistoryInspector.cs
+++ /dev/null
@@ -1,178 +0,0 @@
-using System.Runtime.CompilerServices;
-using System.Text;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Threading;
-using ArIED61850Tester.Models;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// Makes the native FAT report preview useful as an operator evidence/history inspector.
-/// The immutable report snapshot remains the preview/export authority; this enhancement
-/// only appends the persisted chronological audit trail for the selected IEC identity.
-/// </summary>
-public partial class MainWindow
-{
-    private DispatcherTimer? _nativeFatHistoryInspectorInstallRetry;
-    private ScrollViewer? _nativeFatHistoryScrollViewer;
-    private bool _nativeFatHistoryInspectorAttached;
-    private bool _nativeFatHistoryRenderQueued;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatHistoryInspector()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatHistoryInspector_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatHistoryInspector_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatHistoryInspectorAttached)
-            return;
-
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryAttachNativeFatHistoryInspector));
-    }
-
-    private void TryAttachNativeFatHistoryInspector()
-    {
-        if (_nativeFatHistoryInspectorAttached || !IsLoaded)
-            return;
-
-        if (!_nativeFatReportPreviewEnhanced ||
-            _nativeFatPreviewPane?.Child is not Grid previewRoot ||
-            _nativeFatPreviewGrid == null ||
-            _nativeFatPreviewEvidenceText == null)
-        {
-            _nativeFatHistoryInspectorInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
-            {
-                Interval = TimeSpan.FromMilliseconds(180)
-            };
-            _nativeFatHistoryInspectorInstallRetry.Tick -= NativeFatHistoryInspectorInstallRetry_Tick;
-            _nativeFatHistoryInspectorInstallRetry.Tick += NativeFatHistoryInspectorInstallRetry_Tick;
-            _nativeFatHistoryInspectorInstallRetry.Start();
-            return;
-        }
-
-        _nativeFatHistoryInspectorInstallRetry?.Stop();
-        _nativeFatHistoryInspectorAttached = true;
-
-        // The evidence block can become long once retest history exists. Keep the report
-        // pane compact and give only the evidence/history area its own vertical scrolling.
-        if (previewRoot.Children.Contains(_nativeFatPreviewEvidenceText))
-            previewRoot.Children.Remove(_nativeFatPreviewEvidenceText);
-
-        _nativeFatPreviewEvidenceText.Margin = new Thickness(0);
-        _nativeFatPreviewEvidenceText.TextWrapping = TextWrapping.Wrap;
-        _nativeFatHistoryScrollViewer = new ScrollViewer
-        {
-            Content = _nativeFatPreviewEvidenceText,
-            Margin = new Thickness(0, 8, 0, 0),
-            MaxHeight = 190,
-            VerticalScrollBarVisibility = ScrollBarVisibility.Auto,
-            HorizontalScrollBarVisibility = ScrollBarVisibility.Disabled,
-            CanContentScroll = false
-        };
-        Grid.SetRow(_nativeFatHistoryScrollViewer, 5);
-        previewRoot.Children.Add(_nativeFatHistoryScrollViewer);
-
-        _nativeFatPreviewGrid.SelectionChanged += NativeFatHistoryInspector_SelectionChanged;
-        Closed += NativeFatHistoryInspector_MainWindowClosed;
-        QueueNativeFatHistoryInspectorRender();
-    }
-
-    private void NativeFatHistoryInspectorInstallRetry_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatHistoryInspectorInstallRetry?.Stop();
-        TryAttachNativeFatHistoryInspector();
-    }
-
-    private void NativeFatHistoryInspector_SelectionChanged(object sender, SelectionChangedEventArgs e)
-        => QueueNativeFatHistoryInspectorRender();
-
-    private void QueueNativeFatHistoryInspectorRender()
-    {
-        if (!_nativeFatHistoryInspectorAttached || _nativeFatHistoryRenderQueued)
-            return;
-
-        _nativeFatHistoryRenderQueued = true;
-        Dispatcher.BeginInvoke(
-            DispatcherPriority.ContextIdle,
-            new Action(() =>
-            {
-                _nativeFatHistoryRenderQueued = false;
-                RenderNativeFatHistoryInspector();
-            }));
-    }
-
-    private void RenderNativeFatHistoryInspector()
-    {
-        if (_nativeFatPreviewEvidenceText == null)
-            return;
-
-        if (_nativeFatPreviewGrid?.SelectedItem is not NativeFatReportRow reportRow)
-        {
-            _nativeFatPreviewEvidenceText.Text = "Select a report row to inspect captured evidence and retest history.";
-            return;
-        }
-
-        var text = new StringBuilder(reportRow.EvidenceSummaryText);
-        var state = _nativeFatCurrentState;
-        var key = NativeFatIdentity.BuildKey(reportRow.IecReference, reportRow.FunctionalConstraint);
-        var persisted = state?.Signals.FirstOrDefault(signal =>
-            signal.Key.Equals(key, StringComparison.OrdinalIgnoreCase));
-        var history = persisted?.History ?? new List<NativeFatHistoryEntry>();
-
-        text.Append("\n\nHISTORY · latest first");
-        if (history.Count == 0)
-        {
-            text.Append("\nNo previous capture/result transitions recorded.");
-        }
-        else
-        {
-            foreach (var entry in history
-                         .OrderByDescending(item => item.TimestampUtc)
-                         .Take(8))
-            {
-                text.Append("\n")
-                    .Append(entry.TimestampUtc.ToLocalTime().ToString("yyyy-MM-dd HH:mm:ss"))
-                    .Append("  ·  ")
-                    .Append(string.IsNullOrWhiteSpace(entry.Action) ? "State update" : entry.Action.Trim());
-
-                if (!string.IsNullOrWhiteSpace(entry.Result))
-                    text.Append("  ·  ").Append(entry.Result.Trim());
-                if (entry.Value1 != null)
-                    text.Append("  ·  V1=").Append(entry.Value1.Value);
-                if (entry.Value2 != null)
-                    text.Append("  ·  V2=").Append(entry.Value2.Value);
-                if (!string.IsNullOrWhiteSpace(entry.Note))
-                    text.Append("  ·  ").Append(entry.Note.Trim());
-            }
-
-            if (history.Count > 8)
-                text.Append("\n+").Append(history.Count - 8).Append(" earlier record(s) retained in the per-IED JSON.");
-        }
-
-        if (reportRow.IsHistorical)
-            text.Append("\n\nThis IEC identity is historical: it is no longer in the current Explorer scope, but its FAT evidence is retained.");
-
-        _nativeFatPreviewEvidenceText.Text = text.ToString();
-        _nativeFatHistoryScrollViewer?.ScrollToTop();
-    }
-
-    private void NativeFatHistoryInspector_MainWindowClosed(object? sender, EventArgs e)
-    {
-        _nativeFatHistoryInspectorInstallRetry?.Stop();
-        if (_nativeFatPreviewGrid != null)
-            _nativeFatPreviewGrid.SelectionChanged -= NativeFatHistoryInspector_SelectionChanged;
-        Closed -= NativeFatHistoryInspector_MainWindowClosed;
-        _nativeFatHistoryInspectorAttached = false;
-        _nativeFatHistoryRenderQueued = false;
-        _nativeFatHistoryScrollViewer = null;
-    }
-}
diff --git a/MainWindow.NativeFatPersistenceSafety.cs b/MainWindow.NativeFatPersistenceSafety.cs
deleted file mode 100644
index 4f7e192b2..000000000
--- a/MainWindow.NativeFatPersistenceSafety.cs
+++ /dev/null
@@ -1,120 +0,0 @@
-using System.ComponentModel;
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Threading;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// Persistence durability guard for native FAT.
-///
-/// The normal 350 ms autosave debounce keeps multi-row operations cheap, but two short
-/// windows still need explicit protection:
-/// 1) the operator switches IED before the debounce fires; and
-/// 2) the operator closes ARSAS immediately after a capture/result change.
-///
-/// Native FAT caches one state object per stable IED. Flush inactive cached states after
-/// a SelectedDevice change and flush every cached state synchronously during Closing so
-/// evidence from the previously selected relay cannot be stranded in memory.
-/// </summary>
-public partial class MainWindow
-{
-    private bool _nativeFatPersistenceSafetyAttached;
-    private bool _nativeFatSwitchFlushQueued;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatPersistenceSafety()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatPersistenceSafety_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatPersistenceSafety_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatPersistenceSafetyAttached)
-            return;
-
-        window._nativeFatPersistenceSafetyAttached = true;
-        window.PropertyChanged += window.NativeFatPersistenceSafety_PropertyChanged;
-        window.Closing += window.NativeFatPersistenceSafety_Closing;
-        window.Closed += window.NativeFatPersistenceSafety_Closed;
-    }
-
-    private void NativeFatPersistenceSafety_PropertyChanged(object? sender, PropertyChangedEventArgs e)
-    {
-        if (e.PropertyName != nameof(SelectedDevice) || !_nativeFatInstalled || _nativeFatSwitchFlushQueued)
-            return;
-
-        // Let the normal SelectedDevice handler finish rebinding the workspace first.
-        // The previous state remains in _nativeFatStateCache, so a ContextIdle flush can
-        // save it without blocking the IED switch or relying on _nativeFatCurrentState.
-        _nativeFatSwitchFlushQueued = true;
-        Dispatcher.BeginInvoke(
-            DispatcherPriority.ContextIdle,
-            new Action(() =>
-            {
-                _nativeFatSwitchFlushQueued = false;
-                _ = NativeFatPersistenceSafety_FlushInactiveStatesAsync();
-            }));
-    }
-
-    private async Task NativeFatPersistenceSafety_FlushInactiveStatesAsync()
-    {
-        if (_nativeFatStateCache.Count == 0)
-            return;
-
-        var current = _nativeFatCurrentState;
-        var inactive = _nativeFatStateCache.Values
-            .Where(state => !ReferenceEquals(state, current))
-            .Distinct()
-            .ToArray();
-
-        foreach (var state in inactive)
-            await SaveNativeFatStateAsync(state);
-    }
-
-    private void NativeFatPersistenceSafety_Closing(object? sender, CancelEventArgs e)
-    {
-        _nativeFatSaveTimer?.Stop();
-
-        var states = _nativeFatStateCache.Values.Distinct().ToList();
-        if (_nativeFatCurrentState != null && !states.Contains(_nativeFatCurrentState))
-            states.Add(_nativeFatCurrentState);
-        if (states.Count == 0)
-            return;
-
-        foreach (var state in states)
-        {
-            try
-            {
-                // Intentionally bypass the UI save gate here. A debounced asynchronous
-                // save may currently own that gate and need the dispatcher for its
-                // continuation; waiting for the gate synchronously could deadlock Closing.
-                // NativeFatStateStore uses unique temp files + atomic replace, so a
-                // concurrent same-state flush is safe and never truncates the valid file.
-                _nativeFatStore.SaveAsync(state, CancellationToken.None).GetAwaiter().GetResult();
-            }
-            catch (Exception ex)
-            {
-                // Shutdown must remain possible. Any previous valid JSON is preserved;
-                // record the specific IED failure while diagnostics are still available.
-                AddLog(
-                    "WARN",
-                    "Native FAT",
-                    $"Final FAT persistence flush failed for {state.IedName}: {ex.Message}");
-            }
-        }
-    }
-
-    private void NativeFatPersistenceSafety_Closed(object? sender, EventArgs e)
-    {
-        PropertyChanged -= NativeFatPersistenceSafety_PropertyChanged;
-        Closing -= NativeFatPersistenceSafety_Closing;
-        Closed -= NativeFatPersistenceSafety_Closed;
-        _nativeFatPersistenceSafetyAttached = false;
-        _nativeFatSwitchFlushQueued = false;
-    }
-}
diff --git a/MainWindow.NativeFatReportPreview.cs b/MainWindow.NativeFatReportPreview.cs
deleted file mode 100644
index bda0a5a72..000000000
--- a/MainWindow.NativeFatReportPreview.cs
+++ /dev/null
@@ -1,279 +0,0 @@
-using System.Collections.Specialized;
-using System.ComponentModel;
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Data;
-using System.Windows.Media;
-using System.Windows.Threading;
-using ArIED61850Tester.Models;
-using ArIED61850Tester.Services;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// P2 report-preview hardening for the native FAT workspace. The preview consumes an
-/// immutable evidence snapshot instead of the live row objects, while the main FAT grid
-/// remains directly bound to Explorer/runtime values for testing.
-/// </summary>
-public partial class MainWindow
-{
-    private DispatcherTimer? _nativeFatReportPreviewInstallRetry;
-    private TextBlock? _nativeFatPreviewEvidenceText;
-    private NativeFatReportSnapshot? _nativeFatReportSnapshot;
-    private readonly HashSet<NativeFatSignalRow> _nativeFatReportObservedRows = new();
-    private bool _nativeFatReportPreviewEnhanced;
-    private bool _nativeFatReportRefreshQueued;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatReportPreviewEnhancement()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatReportPreview_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatReportPreview_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatReportPreviewEnhanced)
-            return;
-
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryInstallNativeFatReportPreviewEnhancement));
-    }
-
-    private void TryInstallNativeFatReportPreviewEnhancement()
-    {
-        if (_nativeFatReportPreviewEnhanced || !IsLoaded)
-            return;
-
-        if (!_nativeFatInstalled || _nativeFatPreviewPane?.Child is not Grid previewRoot ||
-            _nativeFatPreviewGrid == null || _nativeFatPreviewSummaryText == null)
-        {
-            _nativeFatReportPreviewInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
-            {
-                Interval = TimeSpan.FromMilliseconds(180)
-            };
-            _nativeFatReportPreviewInstallRetry.Tick -= NativeFatReportPreviewInstallRetry_Tick;
-            _nativeFatReportPreviewInstallRetry.Tick += NativeFatReportPreviewInstallRetry_Tick;
-            _nativeFatReportPreviewInstallRetry.Start();
-            return;
-        }
-
-        _nativeFatReportPreviewInstallRetry?.Stop();
-        _nativeFatReportPreviewEnhanced = true;
-
-        // The initial P2 pane already has title/device/summary/grid/persistence rows. Add
-        // one compact evidence inspector between the grid and persistence note.
-        previewRoot.RowDefinitions.Insert(5, new RowDefinition { Height = GridLength.Auto });
-        if (_nativeFatPersistenceText != null)
-            Grid.SetRow(_nativeFatPersistenceText, 6);
-
-        _nativeFatPreviewEvidenceText = new TextBlock
-        {
-            Text = "Select a report row to inspect captured evidence.",
-            Margin = new Thickness(0, 8, 0, 0),
-            Padding = new Thickness(8, 7, 8, 7),
-            FontSize = 9.4,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B)),
-            Background = new SolidColorBrush(Color.FromRgb(0xF7, 0xF9, 0xFC)),
-            TextWrapping = TextWrapping.Wrap
-        };
-        Grid.SetRow(_nativeFatPreviewEvidenceText, 5);
-        previewRoot.Children.Add(_nativeFatPreviewEvidenceText);
-
-        // Existing preview columns deliberately use property names also exposed by the
-        // snapshot row. Add the canonical IEC identity so report scope can be audited.
-        if (_nativeFatPreviewGrid.Columns.All(column => !Equals(column.Header, "IEC")))
-        {
-            _nativeFatPreviewGrid.Columns.Insert(1, new DataGridTextColumn
-            {
-                Header = "IEC",
-                Binding = new Binding(nameof(NativeFatReportRow.IecReference)) { Mode = BindingMode.OneWay },
-                Width = new DataGridLength(1.15, DataGridLengthUnitType.Star),
-                MinWidth = 105
-            });
-        }
-
-        if (_nativeFatPreviewGrid.Columns.Count >= 5)
-        {
-            _nativeFatPreviewGrid.Columns[0].Width = new DataGridLength(1.15, DataGridLengthUnitType.Star);
-            _nativeFatPreviewGrid.Columns[2].Width = new DataGridLength(0.58, DataGridLengthUnitType.Star);
-            _nativeFatPreviewGrid.Columns[3].Width = new DataGridLength(0.58, DataGridLengthUnitType.Star);
-            _nativeFatPreviewGrid.Columns[4].Width = new DataGridLength(0.62, DataGridLengthUnitType.Star);
-        }
-
-        _nativeFatPreviewGrid.SelectionChanged += NativeFatPreviewGrid_SelectionChanged;
-        _nativeFatPreviewPane!.IsVisibleChanged += NativeFatPreviewPane_IsVisibleChanged;
-        _nativeFatRows.CollectionChanged += NativeFatReportRows_CollectionChanged;
-        if (_nativeFatShowHistoricalCheck != null)
-        {
-            _nativeFatShowHistoricalCheck.Checked += NativeFatReportScope_Changed;
-            _nativeFatShowHistoricalCheck.Unchecked += NativeFatReportScope_Changed;
-        }
-
-        SyncNativeFatReportRowSubscriptions();
-        Closed += NativeFatReportPreview_MainWindowClosed;
-        QueueNativeFatReportPreviewRefresh();
-    }
-
-    private void NativeFatReportPreviewInstallRetry_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatReportPreviewInstallRetry?.Stop();
-        TryInstallNativeFatReportPreviewEnhancement();
-    }
-
-    private void NativeFatPreviewPane_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
-    {
-        if (_nativeFatPreviewPane?.IsVisible == true && _nativeFatPreviewColumn != null)
-        {
-            // 330 px was enough for a four-column mock preview but not for auditable IEC
-            // identity + evidence. Keep it compact while making the report pane useful.
-            _nativeFatPreviewColumn.Width = new GridLength(430);
-            QueueNativeFatReportPreviewRefresh();
-        }
-    }
-
-    private void NativeFatReportRows_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
-    {
-        SyncNativeFatReportRowSubscriptions();
-        QueueNativeFatReportPreviewRefresh();
-    }
-
-    private void SyncNativeFatReportRowSubscriptions()
-    {
-        var current = _nativeFatRows.ToHashSet();
-        foreach (var row in _nativeFatReportObservedRows.Where(row => !current.Contains(row)).ToArray())
-        {
-            row.PropertyChanged -= NativeFatReportRow_PropertyChanged;
-            _nativeFatReportObservedRows.Remove(row);
-        }
-
-        foreach (var row in current)
-        {
-            if (!_nativeFatReportObservedRows.Add(row))
-                continue;
-            row.PropertyChanged += NativeFatReportRow_PropertyChanged;
-        }
-    }
-
-    private void NativeFatReportRow_PropertyChanged(object? sender, PropertyChangedEventArgs e)
-    {
-        if (e.PropertyName is nameof(NativeFatSignalRow.Value1Text) or
-            nameof(NativeFatSignalRow.Value2Text) or
-            nameof(NativeFatSignalRow.Result) or
-            nameof(NativeFatSignalRow.HistoryText) or
-            nameof(NativeFatSignalRow.IsHistorical))
-        {
-            QueueNativeFatReportPreviewRefresh();
-        }
-    }
-
-    private void NativeFatReportScope_Changed(object sender, RoutedEventArgs e)
-        => QueueNativeFatReportPreviewRefresh();
-
-    private void QueueNativeFatReportPreviewRefresh()
-    {
-        if (!_nativeFatReportPreviewEnhanced || _nativeFatPreviewPane?.IsVisible != true || _nativeFatReportRefreshQueued)
-            return;
-
-        _nativeFatReportRefreshQueued = true;
-        Dispatcher.BeginInvoke(
-            DispatcherPriority.ContextIdle,
-            new Action(() =>
-            {
-                _nativeFatReportRefreshQueued = false;
-                RefreshNativeFatReportPreviewSnapshot();
-            }));
-    }
-
-    private void RefreshNativeFatReportPreviewSnapshot()
-    {
-        if (_nativeFatPreviewPane?.IsVisible != true || _nativeFatPreviewGrid == null)
-            return;
-
-        var device = SelectedDevice;
-        var state = _nativeFatCurrentState;
-        if (device == null || state == null ||
-            !state.DeviceId.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase))
-        {
-            _nativeFatReportSnapshot = null;
-            _nativeFatPreviewGrid.ItemsSource = null;
-            if (_nativeFatPreviewSummaryText != null)
-                _nativeFatPreviewSummaryText.Text = "Select an IED with a loaded FAT state.";
-            if (_nativeFatPreviewEvidenceText != null)
-                _nativeFatPreviewEvidenceText.Text = "No report evidence is loaded.";
-            return;
-        }
-
-        var previousReference = (_nativeFatPreviewGrid.SelectedItem as NativeFatReportRow)?.IecReference;
-        _nativeFatReportSnapshot = NativeFatReportSnapshotBuilder.Build(device, state);
-        var showHistorical = _nativeFatShowHistoricalCheck?.IsChecked == true;
-        var visibleRows = _nativeFatReportSnapshot.Rows
-            .Where(row => showHistorical || !row.IsHistorical)
-            .ToArray();
-        _nativeFatPreviewGrid.ItemsSource = visibleRows;
-
-        if (_nativeFatPreviewDeviceText != null)
-            _nativeFatPreviewDeviceText.Text = $"IED · {_nativeFatReportSnapshot.IedName} · {_nativeFatReportSnapshot.IpAddress}:{_nativeFatReportSnapshot.Port}";
-        if (_nativeFatPreviewSummaryText != null)
-        {
-            var scopeSuffix = !showHistorical && _nativeFatReportSnapshot.HistoricalCount > 0
-                ? " · historical hidden"
-                : string.Empty;
-            _nativeFatPreviewSummaryText.Text = _nativeFatReportSnapshot.SummaryText + scopeSuffix;
-        }
-
-        NativeFatReportRow? selection = null;
-        if (!string.IsNullOrWhiteSpace(previousReference))
-        {
-            selection = visibleRows.FirstOrDefault(row =>
-                row.IecReference.Equals(previousReference, StringComparison.OrdinalIgnoreCase));
-        }
-        selection ??= visibleRows.FirstOrDefault();
-        if (selection != null)
-        {
-            _nativeFatPreviewGrid.SelectedItem = selection;
-            _nativeFatPreviewGrid.ScrollIntoView(selection);
-        }
-        else
-        {
-            UpdateNativeFatPreviewEvidence(null);
-        }
-    }
-
-    private void NativeFatPreviewGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
-        => UpdateNativeFatPreviewEvidence(_nativeFatPreviewGrid?.SelectedItem as NativeFatReportRow);
-
-    private void UpdateNativeFatPreviewEvidence(NativeFatReportRow? row)
-    {
-        if (_nativeFatPreviewEvidenceText == null)
-            return;
-
-        _nativeFatPreviewEvidenceText.Text = row?.EvidenceSummaryText ??
-                                             "Select a report row to inspect captured evidence.";
-    }
-
-    private void NativeFatReportPreview_MainWindowClosed(object? sender, EventArgs e)
-    {
-        _nativeFatReportPreviewInstallRetry?.Stop();
-        if (_nativeFatPreviewGrid != null)
-            _nativeFatPreviewGrid.SelectionChanged -= NativeFatPreviewGrid_SelectionChanged;
-        if (_nativeFatPreviewPane != null)
-            _nativeFatPreviewPane.IsVisibleChanged -= NativeFatPreviewPane_IsVisibleChanged;
-        _nativeFatRows.CollectionChanged -= NativeFatReportRows_CollectionChanged;
-        if (_nativeFatShowHistoricalCheck != null)
-        {
-            _nativeFatShowHistoricalCheck.Checked -= NativeFatReportScope_Changed;
-            _nativeFatShowHistoricalCheck.Unchecked -= NativeFatReportScope_Changed;
-        }
-
-        foreach (var row in _nativeFatReportObservedRows)
-            row.PropertyChanged -= NativeFatReportRow_PropertyChanged;
-        _nativeFatReportObservedRows.Clear();
-        Closed -= NativeFatReportPreview_MainWindowClosed;
-    }
-}
diff --git a/MainWindow.NativeFatWorkspace.cs b/MainWindow.NativeFatWorkspace.cs
index 3c28770ea..e5d4f545e 100644
--- a/MainWindow.NativeFatWorkspace.cs
+++ b/MainWindow.NativeFatWorkspace.cs
@@ -1,980 +1,28 @@
-using System.Collections.ObjectModel;
-using System.Collections.Specialized;
-using System.ComponentModel;
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Controls.Primitives;
-using System.Windows.Data;
-using System.Windows.Input;
-using System.Windows.Media;
-using System.Windows.Media.Animation;
+using System;
 using System.Windows.Threading;
-using ArIED61850Tester.Models;
-using ArIED61850Tester.Services;
 
 namespace ArIED61850Tester;
 
 /// <summary>
-/// Native continuous FAT workspace layered on the persistent P0/P1 Engineering shell.
+/// M7 compatibility bridge for the canonical seventh Engineering destination.
 ///
-/// Authority boundary:
-/// - IED Explorer owns SelectedDevice, signal engineering, live value and acquisition.
-/// - the shared Command Dock remains the only command UI/runtime owner.
-/// - this workspace owns only FAT captures, result/history and per-IED persistence.
-///
-/// The legacy IoListTestingWindow remains untouched as a compatibility fallback while
-/// report/evidence parity is migrated incrementally.
+/// MainWindow.xaml owns the FAT tab and navigation button. Production FAT is mounted
+/// into that permanent XAML slot by MainWindow.ProductionFatTab.cs. This file deliberately
+/// owns no FAT rows, capture commands, evidence state, persistence, preview, export, or timers.
+/// It retains only the shared slot index and a deferred navigation-geometry refresh used by
+/// the canonical MainWindow shell.
 /// </summary>
 public partial class MainWindow
 {
     private const int NativeFatWorkspaceIndex = 6;
 
-    private readonly NativeFatStateStore _nativeFatStore = new();
-    private readonly ObservableCollection<NativeFatSignalRow> _nativeFatRows = new();
-    private readonly Dictionary<string, NativeFatDeviceState> _nativeFatStateCache = new(StringComparer.OrdinalIgnoreCase);
-    private readonly SemaphoreSlim _nativeFatSaveGate = new(1, 1);
-
-    private DispatcherTimer? _nativeFatInstallRetry;
-    private DispatcherTimer? _nativeFatReconcileTimer;
-    private DispatcherTimer? _nativeFatSaveTimer;
-    private CancellationTokenSource? _nativeFatLoadCts;
-    private TabItem? _nativeFatTab;
-    private Button? _nativeFatNavButton;
-    private DataGrid? _nativeFatGrid;
-    private DataGrid? _nativeFatPreviewGrid;
-    private ICollectionView? _nativeFatView;
-    private TextBox? _nativeFatSearchBox;
-    private CheckBox? _nativeFatShowHistoricalCheck;
-    private Border? _nativeFatPreviewPane;
-    private ColumnDefinition? _nativeFatPreviewGapColumn;
-    private ColumnDefinition? _nativeFatPreviewColumn;
-    private TextBlock? _nativeFatContextText;
-    private TextBlock? _nativeFatStatusText;
-    private TextBlock? _nativeFatPreviewDeviceText;
-    private TextBlock? _nativeFatPreviewSummaryText;
-    private TextBlock? _nativeFatPersistenceText;
-    private Iec61850MonitorDevice? _nativeFatObservedDevice;
-    private NativeFatDeviceState? _nativeFatCurrentState;
-    private bool _nativeFatInstalled;
-    private bool _nativeFatLoading;
-    private bool _nativeFatPreviewVisible;
-
-    [ModuleInitializer]
-    internal static void RegisterNativeFatWorkspace()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(NativeFatWorkspace_MainWindowLoaded),
-            handledEventsToo: true);
-    }
-
-    private static void NativeFatWorkspace_MainWindowLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || window._nativeFatInstalled)
-            return;
-
-        // P0 moves MainTabs/Explorer/Command Dock at ContextIdle. Install FAT after that
-        // re-parenting has settled so this stacked feature never races the stable P0 shell.
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryInstallNativeFatWorkspace));
-    }
-
-    private void TryInstallNativeFatWorkspace()
-    {
-        if (_nativeFatInstalled || !IsLoaded)
-            return;
-
-        if (_persistentWorkbench == null || MainTabs.Items.Count < 6)
-        {
-            _nativeFatInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
-            {
-                Interval = TimeSpan.FromMilliseconds(140)
-            };
-            _nativeFatInstallRetry.Tick -= NativeFatInstallRetry_Tick;
-            _nativeFatInstallRetry.Tick += NativeFatInstallRetry_Tick;
-            _nativeFatInstallRetry.Start();
-            return;
-        }
-
-        _nativeFatInstallRetry?.Stop();
-        _nativeFatInstalled = true;
-
-        _nativeFatTab = new TabItem
-        {
-            Header = "FAT",
-            Content = BuildNativeFatWorkspaceContent()
-        };
-        MainTabs.Items.Add(_nativeFatTab);
-
-        // The command dock is deliberately shared, not cloned. FAT is command-centric,
-        // therefore start with the same expanded behavior as Explorer/Event Log.
-        _persistentWorkbench.DockExpandedByWorkspace[NativeFatWorkspaceIndex] = true;
-
-        InstallNativeFatNavigationButton();
-        InstallNativeFatTimers();
-        AttachNativeFatObservedDevice(SelectedDevice);
-
-        PropertyChanged += NativeFat_MainWindowPropertyChanged;
-        MainTabs.SelectionChanged += NativeFat_MainTabsSelectionChanged;
-        SizeChanged += NativeFat_MainWindowSizeChanged;
-        Closed += NativeFat_MainWindowClosed;
-
-        QueueNativeFatNavigationGeometry();
-        UpdateNativeFatSummary();
-    }
-
-    private void NativeFatInstallRetry_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatInstallRetry?.Stop();
-        TryInstallNativeFatWorkspace();
-    }
-
-    private UIElement BuildNativeFatWorkspaceContent()
-    {
-        var root = new Grid { Margin = new Thickness(0) };
-        root.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-        root.RowDefinitions.Add(new RowDefinition { Height = new GridLength(8) });
-        root.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-        root.RowDefinitions.Add(new RowDefinition { Height = new GridLength(8) });
-        root.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
-
-        var header = BuildNativeFatHeader();
-        Grid.SetRow(header, 0);
-        root.Children.Add(header);
-
-        var toolbar = BuildNativeFatToolbar();
-        Grid.SetRow(toolbar, 2);
-        root.Children.Add(toolbar);
-
-        var body = BuildNativeFatBody();
-        Grid.SetRow(body, 4);
-        root.Children.Add(body);
-        return root;
-    }
-
-    private Border BuildNativeFatHeader()
-    {
-        var border = new Border
-        {
-            Padding = new Thickness(14, 10, 14, 10),
-            CornerRadius = new CornerRadius(12),
-            Background = ResourceBrush("SurfaceElevated", Colors.White),
-            BorderBrush = ResourceBrush("Line", Color.FromRgb(0xDD, 0xE5, 0xF0)),
-            BorderThickness = new Thickness(1)
-        };
-
-        var grid = new Grid();
-        grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
-        grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });
-
-        var titleStack = new StackPanel();
-        titleStack.Children.Add(new TextBlock
-        {
-            Text = "FAT · Continuous Testing",
-            FontSize = 14.5,
-            FontWeight = FontWeights.SemiBold,
-            Foreground = ResourceBrush("Ink", Color.FromRgb(0x20, 0x30, 0x4A))
-        });
-        titleStack.Children.Add(new TextBlock
-        {
-            Text = "Explorer signal authority · live values stay shared · captures autosave per IED · removed signals keep their history",
-            Margin = new Thickness(0, 3, 0, 0),
-            FontSize = 10.8,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B)),
-            TextWrapping = TextWrapping.Wrap
-        });
-        grid.Children.Add(titleStack);
-
-        _nativeFatContextText = new TextBlock
-        {
-            Text = "IED · NONE",
-            FontSize = 10.5,
-            FontWeight = FontWeights.SemiBold,
-            Foreground = ResourceBrush("Accent", Color.FromRgb(0x25, 0x63, 0xEB)),
-            VerticalAlignment = VerticalAlignment.Center,
-            Margin = new Thickness(14, 0, 0, 0)
-        };
-        Grid.SetColumn(_nativeFatContextText, 1);
-        grid.Children.Add(_nativeFatContextText);
-
-        border.Child = grid;
-        return border;
-    }
-
-    private Border BuildNativeFatToolbar()
-    {
-        var border = new Border
-        {
-            Padding = new Thickness(9, 7, 9, 7),
-            CornerRadius = new CornerRadius(11),
-            Background = ResourceBrush("Surface", Colors.White),
-            BorderBrush = ResourceBrush("Line", Color.FromRgb(0xDD, 0xE5, 0xF0)),
-            BorderThickness = new Thickness(1)
-        };
-
-        var wrap = new WrapPanel
-        {
-            Orientation = Orientation.Horizontal,
-            VerticalAlignment = VerticalAlignment.Center
-        };
-
-        _nativeFatSearchBox = new TextBox
-        {
-            Width = 210,
-            Height = 30,
-            Margin = new Thickness(0, 0, 8, 0),
-            Padding = new Thickness(8, 4, 8, 4),
-            VerticalContentAlignment = VerticalAlignment.Center,
-            ToolTip = "Search Signal, IEC reference, type, value, result or history state"
-        };
-        _nativeFatSearchBox.TextChanged += NativeFatSearchBox_TextChanged;
-        wrap.Children.Add(_nativeFatSearchBox);
-
-        wrap.Children.Add(CreateNativeFatButton("Refresh", NativeFatRefresh_Click, "Reconcile the saved FAT state with the current Explorer signal scope."));
-        wrap.Children.Add(CreateNativeFatButton("Capture V1", NativeFatCapture1_Click, "Capture the current Explorer live value into Value 1 for selected FAT rows."));
-        wrap.Children.Add(CreateNativeFatButton("Capture V2", NativeFatCapture2_Click, "Capture the current Explorer live value into Value 2 for selected FAT rows."));
-        wrap.Children.Add(CreateNativeFatButton("PASS", NativeFatPass_Click, "Mark selected FAT rows PASS; previous results remain in history."));
-        wrap.Children.Add(CreateNativeFatButton("REVIEW", NativeFatReview_Click, "Mark selected FAT rows REVIEW; previous results remain in history."));
-        wrap.Children.Add(CreateNativeFatButton("FAIL", NativeFatFail_Click, "Mark selected FAT rows FAIL; previous results remain in history."));
-        wrap.Children.Add(CreateNativeFatButton("Reset current", NativeFatReset_Click, "Clear current captures/result while retaining the previous state in history."));
-        wrap.Children.Add(CreateNativeFatButton("Report Preview", NativeFatReportPreview_Click, "Show or hide the native FAT report preview without opening another FAT runtime."));
-
-        _nativeFatShowHistoricalCheck = new CheckBox
-        {
-            Content = "Show historical",
-            IsChecked = false,
-            Margin = new Thickness(7, 6, 7, 0),
-            VerticalAlignment = VerticalAlignment.Center,
-            FontSize = 10.7,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B)),
-            ToolTip = "Show signals removed from the current Explorer/SCL scope. Their previous FAT evidence is never deleted."
-        };
-        _nativeFatShowHistoricalCheck.Checked += NativeFatShowHistorical_Changed;
-        _nativeFatShowHistoricalCheck.Unchecked += NativeFatShowHistorical_Changed;
-        wrap.Children.Add(_nativeFatShowHistoricalCheck);
-
-        _nativeFatStatusText = new TextBlock
-        {
-            Text = "Select an IED to begin.",
-            Margin = new Thickness(9, 7, 0, 0),
-            VerticalAlignment = VerticalAlignment.Center,
-            FontSize = 10.4,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B))
-        };
-        wrap.Children.Add(_nativeFatStatusText);
-
-        border.Child = wrap;
-        return border;
-    }
-
-    private Grid BuildNativeFatBody()
-    {
-        var body = new Grid();
-        body.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star), MinWidth = 0 });
-        _nativeFatPreviewGapColumn = new ColumnDefinition { Width = new GridLength(0) };
-        _nativeFatPreviewColumn = new ColumnDefinition { Width = new GridLength(0), MinWidth = 0 };
-        body.ColumnDefinitions.Add(_nativeFatPreviewGapColumn);
-        body.ColumnDefinitions.Add(_nativeFatPreviewColumn);
-
-        _nativeFatGrid = BuildNativeFatGrid(preview: false);
-        Grid.SetColumn(_nativeFatGrid, 0);
-        body.Children.Add(_nativeFatGrid);
-
-        _nativeFatPreviewPane = BuildNativeFatPreviewPane();
-        _nativeFatPreviewPane.Visibility = Visibility.Collapsed;
-        Grid.SetColumn(_nativeFatPreviewPane, 2);
-        body.Children.Add(_nativeFatPreviewPane);
-        return body;
-    }
-
-    private DataGrid BuildNativeFatGrid(bool preview)
-    {
-        var grid = new DataGrid
-        {
-            AutoGenerateColumns = false,
-            CanUserAddRows = false,
-            CanUserDeleteRows = false,
-            CanUserReorderColumns = true,
-            CanUserResizeColumns = true,
-            HeadersVisibility = DataGridHeadersVisibility.Column,
-            IsReadOnly = true,
-            SelectionMode = preview ? DataGridSelectionMode.Single : DataGridSelectionMode.Extended,
-            SelectionUnit = DataGridSelectionUnit.FullRow,
-            GridLinesVisibility = DataGridGridLinesVisibility.Horizontal,
-            HorizontalGridLinesBrush = ResourceBrush("Line", Color.FromRgb(0xE2, 0xE8, 0xF0)),
-            BorderBrush = ResourceBrush("Line", Color.FromRgb(0xDD, 0xE5, 0xF0)),
-            BorderThickness = new Thickness(1),
-            RowHeight = preview ? 29 : 32,
-            ColumnHeaderHeight = 31,
-            Background = ResourceBrush("Surface", Colors.White),
-            AlternatingRowBackground = new SolidColorBrush(Color.FromRgb(0xFA, 0xFB, 0xFD)),
-            EnableRowVirtualization = true,
-            EnableColumnVirtualization = true
-        };
-
-        if (TryFindResource("DataGridHeaderCompact") is Style headerStyle)
-            grid.ColumnHeaderStyle = headerStyle;
-        if (TryFindResource("DataGridCellCompact") is Style cellStyle)
-            grid.CellStyle = cellStyle;
-
-        var rowStyle = new Style(typeof(DataGridRow));
-        rowStyle.Setters.Add(new Setter(Control.FontSizeProperty, preview ? 10.0 : 10.4));
-        var historicalTrigger = new DataTrigger
-        {
-            Binding = new Binding(nameof(NativeFatSignalRow.IsHistorical)),
-            Value = true
-        };
-        historicalTrigger.Setters.Add(new Setter(UIElement.OpacityProperty, 0.56));
-        rowStyle.Triggers.Add(historicalTrigger);
-        grid.RowStyle = rowStyle;
-
-        if (!preview)
-        {
-            grid.Columns.Add(TextColumn("SIGNAL", nameof(NativeFatSignalRow.SignalName), 1.25, minWidth: 130));
-            grid.Columns.Add(TextColumn("IEC REFERENCE", nameof(NativeFatSignalRow.IecReference), 1.85, minWidth: 190));
-            grid.Columns.Add(TextColumn("TYPE", nameof(NativeFatSignalRow.DataType), 0.72, minWidth: 72));
-            grid.Columns.Add(TextColumn("LIVE VALUE", nameof(NativeFatSignalRow.LiveValue), 0.9, minWidth: 90));
-            grid.Columns.Add(TextColumn("QUALITY", nameof(NativeFatSignalRow.Quality), 0.82, minWidth: 82));
-            grid.Columns.Add(TextColumn("VALUE 1", nameof(NativeFatSignalRow.Value1Text), 0.78, minWidth: 78));
-            grid.Columns.Add(TextColumn("VALUE 2", nameof(NativeFatSignalRow.Value2Text), 0.78, minWidth: 78));
-            grid.Columns.Add(TextColumn("STATUS", nameof(NativeFatSignalRow.StatusText), 0.72, minWidth: 76));
-            grid.Columns.Add(TextColumn("RESULT", nameof(NativeFatSignalRow.Result), 0.72, minWidth: 72));
-            grid.Columns.Add(TextColumn("HISTORY", nameof(NativeFatSignalRow.HistoryText), 0.78, minWidth: 78));
-        }
-        else
-        {
-            grid.Columns.Add(TextColumn("SIGNAL", nameof(NativeFatSignalRow.SignalName), 1.35, minWidth: 115));
-            grid.Columns.Add(TextColumn("V1", nameof(NativeFatSignalRow.Value1Text), 0.65, minWidth: 62));
-            grid.Columns.Add(TextColumn("V2", nameof(NativeFatSignalRow.Value2Text), 0.65, minWidth: 62));
-            grid.Columns.Add(TextColumn("RESULT", nameof(NativeFatSignalRow.Result), 0.72, minWidth: 68));
-        }
-
-        return grid;
-    }
-
-    private Border BuildNativeFatPreviewPane()
-    {
-        var border = new Border
-        {
-            Padding = new Thickness(11),
-            CornerRadius = new CornerRadius(12),
-            Background = ResourceBrush("SurfaceElevated", Colors.White),
-            BorderBrush = ResourceBrush("Line", Color.FromRgb(0xDD, 0xE5, 0xF0)),
-            BorderThickness = new Thickness(1)
-        };
-
-        var grid = new Grid();
-        grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-        grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-        grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-        grid.RowDefinitions.Add(new RowDefinition { Height = new GridLength(8) });
-        grid.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
-        grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
-
-        var title = new TextBlock
-        {
-            Text = "FAT Report Preview",
-            FontSize = 13,
-            FontWeight = FontWeights.SemiBold,
-            Foreground = ResourceBrush("Ink", Color.FromRgb(0x20, 0x30, 0x4A))
-        };
-        grid.Children.Add(title);
-
-        _nativeFatPreviewDeviceText = new TextBlock
-        {
-            Text = "IED · NONE",
-            Margin = new Thickness(0, 4, 0, 0),
-            FontSize = 10.6,
-            FontWeight = FontWeights.SemiBold,
-            Foreground = ResourceBrush("Accent", Color.FromRgb(0x25, 0x63, 0xEB))
-        };
-        Grid.SetRow(_nativeFatPreviewDeviceText, 1);
-        grid.Children.Add(_nativeFatPreviewDeviceText);
-
-        _nativeFatPreviewSummaryText = new TextBlock
-        {
-            Text = "No FAT state loaded.",
-            Margin = new Thickness(0, 4, 0, 0),
-            FontSize = 10.2,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B)),
-            TextWrapping = TextWrapping.Wrap
-        };
-        Grid.SetRow(_nativeFatPreviewSummaryText, 2);
-        grid.Children.Add(_nativeFatPreviewSummaryText);
-
-        _nativeFatPreviewGrid = BuildNativeFatGrid(preview: true);
-        Grid.SetRow(_nativeFatPreviewGrid, 4);
-        grid.Children.Add(_nativeFatPreviewGrid);
-
-        _nativeFatPersistenceText = new TextBlock
-        {
-            Text = "Per-IED JSON · non-destructive reconciliation",
-            Margin = new Thickness(0, 7, 0, 0),
-            FontSize = 9.5,
-            Foreground = ResourceBrush("Muted", Color.FromRgb(0x66, 0x75, 0x8B)),
-            TextWrapping = TextWrapping.Wrap
-        };
-        Grid.SetRow(_nativeFatPersistenceText, 5);
-        grid.Children.Add(_nativeFatPersistenceText);
-
-        border.Child = grid;
-        return border;
-    }
-
-    private DataGridTextColumn TextColumn(string header, string path, double star, double minWidth)
-        => new()
-        {
-            Header = header,
-            Binding = new Binding(path) { Mode = BindingMode.OneWay },
-            Width = new DataGridLength(star, DataGridLengthUnitType.Star),
-            MinWidth = minWidth
-        };
-
-    private Button CreateNativeFatButton(string text, RoutedEventHandler click, string toolTip)
-    {
-        var button = new Button
-        {
-            Content = text,
-            Margin = new Thickness(0, 0, 6, 0),
-            Padding = new Thickness(9, 5, 9, 5),
-            MinHeight = 29,
-            FontSize = 10.5,
-            ToolTip = toolTip
-        };
-        if (TryFindResource("SoftButton") is Style style)
-            button.Style = style;
-        button.Click += click;
-        return button;
-    }
-
-    private void InstallNativeFatNavigationButton()
-    {
-        if (_nativeFatNavButton != null)
-            return;
-
-        while (WorkflowNavGrid.ColumnDefinitions.Count < 7)
-            WorkflowNavGrid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
-
-        Grid.SetColumnSpan(WorkflowPill, 7);
-        _nativeFatNavButton = new Button
-        {
-            Name = "NavNativeFatButton",
-            Content = "FAT",
-            Tag = NativeFatWorkspaceIndex,
-            MinWidth = 0,
-            ToolTip = "Continuous FAT testing using the selected IED and live Explorer signal authority"
-        };
-        if (TryFindResource("SegmentedNavButton") is Style navStyle)
-            _nativeFatNavButton.Style = navStyle;
-        _nativeFatNavButton.Click += NativeFatNavButton_Click;
-        Grid.SetColumn(_nativeFatNavButton, 6);
-        WorkflowNavGrid.Children.Add(_nativeFatNavButton);
-    }
-
-    private void InstallNativeFatTimers()
-    {
-        _nativeFatReconcileTimer = new DispatcherTimer(DispatcherPriority.Background)
-        {
-            Interval = TimeSpan.FromMilliseconds(320)
-        };
-        _nativeFatReconcileTimer.Tick += NativeFatReconcileTimer_Tick;
-
-        _nativeFatSaveTimer = new DispatcherTimer(DispatcherPriority.Background)
-        {
-            Interval = TimeSpan.FromMilliseconds(350)
-        };
-        _nativeFatSaveTimer.Tick += NativeFatSaveTimer_Tick;
-    }
-
-    private async void NativeFatNavButton_Click(object sender, RoutedEventArgs e)
-    {
-        if (MainTabs.Items.Count <= NativeFatWorkspaceIndex)
-            return;
-
-        MainTabs.SelectedIndex = NativeFatWorkspaceIndex;
-        QueueNativeFatNavigationGeometry();
-        await EnsureNativeFatLoadedAsync(forceReconcile: false);
-    }
-
-    private void NativeFat_MainTabsSelectionChanged(object sender, SelectionChangedEventArgs e)
-    {
-        if (!ReferenceEquals(e.Source, MainTabs))
-            return;
-
-        QueueNativeFatNavigationGeometry();
-        if (MainTabs.SelectedIndex == NativeFatWorkspaceIndex)
-            _ = EnsureNativeFatLoadedAsync(forceReconcile: false);
-        else
-            QueueNativeFatSave();
-    }
-
-    private void NativeFat_MainWindowSizeChanged(object sender, SizeChangedEventArgs e)
-        => QueueNativeFatNavigationGeometry();
-
     private void QueueNativeFatNavigationGeometry()
     {
-        if (!_nativeFatInstalled)
+        if (!IsLoaded)
             return;
+
         Dispatcher.BeginInvoke(
             DispatcherPriority.ApplicationIdle,
-            new Action(ApplyNativeFatNavigationGeometry));
-    }
-
-    private void ApplyNativeFatNavigationGeometry()
-    {
-        if (!_nativeFatInstalled || _nativeFatNavButton == null)
-            return;
-
-        var availableWidth = ActualWidth > 0d ? ActualWidth : 1480d;
-        var wide = availableWidth >= 1700d;
-        var medium = availableWidth >= 1380d;
-        var shellWidth = wide ? 1085d : medium ? 995d : 805d;
-
-        WorkflowNavShell.Width = shellWidth;
-        WorkflowNavShell.MinWidth = shellWidth;
-        WorkflowNavShell.Height = 60;
-        WorkflowNavShell.Padding = new Thickness(5, 6, 5, 6);
-        WorkflowNavGrid.ClipToBounds = false;
-
-        while (WorkflowNavGrid.ColumnDefinitions.Count < 7)
-            WorkflowNavGrid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
-        foreach (var column in WorkflowNavGrid.ColumnDefinitions)
-            column.Width = new GridLength(1, GridUnitType.Star);
-
-        var buttons = new[]
-        {
-            NavExplorerButton,
-            NavLiveButton,
-            NavEventsButton,
-            NavAlarmButton,
-            NavGooseButton,
-            NavDiagnosticsButton,
-            _nativeFatNavButton
-        };
-        for (var index = 0; index < buttons.Length; index++)
-        {
-            var button = buttons[index];
-            button.MinHeight = 40;
-            button.MinWidth = 0;
-            button.Margin = new Thickness(1);
-            button.Padding = wide ? new Thickness(9, 7, 9, 7) : new Thickness(5, 7, 5, 7);
-            button.HorizontalContentAlignment = HorizontalAlignment.Center;
-            button.VerticalContentAlignment = VerticalAlignment.Center;
-            button.Foreground = index == MainTabs.SelectedIndex
-                ? Brushes.White
-                : ResourceBrush("Muted", Color.FromRgb(0x5F, 0x6B, 0x7A));
-        }
-
-        Grid.SetColumnSpan(WorkflowPill, 7);
-        var contentWidth = Math.Max(0d, shellWidth - WorkflowNavShell.Padding.Left - WorkflowNavShell.Padding.Right);
-        var cellWidth = contentWidth / 7d;
-        WorkflowPill.Width = Math.Max(1d, cellWidth - 2d);
-        WorkflowPill.Height = 36;
-
-        WorkflowPillTranslate.BeginAnimation(TranslateTransform.XProperty, null);
-        WorkflowPillTranslate.X = Math.Clamp(MainTabs.SelectedIndex, 0, NativeFatWorkspaceIndex) * cellWidth;
-    }
-
-    private void NativeFat_MainWindowPropertyChanged(object? sender, PropertyChangedEventArgs e)
-    {
-        if (e.PropertyName != nameof(SelectedDevice))
-            return;
-
-        QueueNativeFatSave();
-        AttachNativeFatObservedDevice(SelectedDevice);
-        if (MainTabs.SelectedIndex == NativeFatWorkspaceIndex)
-            _ = EnsureNativeFatLoadedAsync(forceReconcile: true);
-        else
-            UpdateNativeFatSummary();
-    }
-
-    private void AttachNativeFatObservedDevice(Iec61850MonitorDevice? device)
-    {
-        if (ReferenceEquals(_nativeFatObservedDevice, device))
-            return;
-
-        if (_nativeFatObservedDevice != null)
-        {
-            _nativeFatObservedDevice.Signals.CollectionChanged -= NativeFatSignalCollectionChanged;
-            _nativeFatObservedDevice.Points.CollectionChanged -= NativeFatSignalCollectionChanged;
-        }
-
-        _nativeFatObservedDevice = device;
-        if (_nativeFatObservedDevice != null)
-        {
-            _nativeFatObservedDevice.Signals.CollectionChanged += NativeFatSignalCollectionChanged;
-            _nativeFatObservedDevice.Points.CollectionChanged += NativeFatSignalCollectionChanged;
-        }
-    }
-
-    private void NativeFatSignalCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
-    {
-        if (MainTabs.SelectedIndex != NativeFatWorkspaceIndex)
-            return;
-        _nativeFatReconcileTimer?.Stop();
-        _nativeFatReconcileTimer?.Start();
-    }
-
-    private async void NativeFatReconcileTimer_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatReconcileTimer?.Stop();
-        await EnsureNativeFatLoadedAsync(forceReconcile: true);
-    }
-
-    private async Task EnsureNativeFatLoadedAsync(bool forceReconcile)
-    {
-        if (!_nativeFatInstalled || MainTabs.SelectedIndex != NativeFatWorkspaceIndex)
-            return;
-
-        var device = SelectedDevice;
-        if (device == null)
-        {
-            ClearNativeFatRows();
-            _nativeFatCurrentState = null;
-            UpdateNativeFatSummary();
-            return;
-        }
-
-        var cacheKey = StableNativeFatDeviceKey(device);
-        if (!forceReconcile && _nativeFatCurrentState != null &&
-            _nativeFatCurrentState.DeviceId.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase))
-        {
-            UpdateNativeFatSummary();
-            return;
-        }
-
-        _nativeFatLoadCts?.Cancel();
-        _nativeFatLoadCts?.Dispose();
-        _nativeFatLoadCts = CancellationTokenSource.CreateLinkedTokenSource(_applicationCancellation.Token);
-        var token = _nativeFatLoadCts.Token;
-        _nativeFatLoading = true;
-        SetNativeFatStatus($"Loading FAT state for {device.Name}…");
-
-        try
-        {
-            NativeFatDeviceState state;
-            if (_nativeFatStateCache.TryGetValue(cacheKey, out var cached))
-            {
-                state = cached;
-            }
-            else
-            {
-                state = await _nativeFatStore.LoadAsync(device, token);
-                token.ThrowIfCancellationRequested();
-                _nativeFatStateCache[cacheKey] = state;
-            }
-
-            // We are back on the WPF dispatcher here. Reconciliation enumerates Explorer
-            // ObservableCollections only on their owning UI thread.
-            token.ThrowIfCancellationRequested();
-            if (!ReferenceEquals(device, SelectedDevice))
-                return;
-
-            NativeFatStateStore.Reconcile(state, device);
-            _nativeFatCurrentState = state;
-            RebuildNativeFatRows(state, device);
-            QueueNativeFatSave();
-            SetNativeFatStatus($"FAT ready · {device.Name}");
-        }
-        catch (OperationCanceledException)
-        {
-            // Fast IED switches are normal; stale loads are intentionally discarded.
-        }
-        catch (Exception ex)
-        {
-            AddLog("WARN", "Native FAT", $"FAT state load/reconcile failed: {ex.Message}");
-            SetNativeFatStatus("FAT state could not be loaded. Explorer monitoring remains unaffected.");
-        }
-        finally
-        {
-            _nativeFatLoading = false;
-            UpdateNativeFatSummary();
-        }
-    }
-
-    private void RebuildNativeFatRows(NativeFatDeviceState state, Iec61850MonitorDevice device)
-    {
-        ClearNativeFatRows();
-        foreach (var row in NativeFatStateStore.BuildRows(state, device))
-        {
-            row.StateChanged += NativeFatRow_StateChanged;
-            _nativeFatRows.Add(row);
-        }
-
-        _nativeFatView = CollectionViewSource.GetDefaultView(_nativeFatRows);
-        _nativeFatView.Filter = NativeFatViewFilter;
-        if (_nativeFatGrid != null)
-            _nativeFatGrid.ItemsSource = _nativeFatView;
-        if (_nativeFatPreviewGrid != null)
-            _nativeFatPreviewGrid.ItemsSource = _nativeFatRows;
-        UpdateNativeFatSummary();
-    }
-
-    private void ClearNativeFatRows()
-    {
-        foreach (var row in _nativeFatRows)
-        {
-            row.StateChanged -= NativeFatRow_StateChanged;
-            row.Dispose();
-        }
-        _nativeFatRows.Clear();
-        _nativeFatView = null;
-        if (_nativeFatGrid != null)
-            _nativeFatGrid.ItemsSource = null;
-        if (_nativeFatPreviewGrid != null)
-            _nativeFatPreviewGrid.ItemsSource = null;
-    }
-
-    private bool NativeFatViewFilter(object item)
-    {
-        if (item is not NativeFatSignalRow row)
-            return false;
-
-        if (row.IsHistorical && _nativeFatShowHistoricalCheck?.IsChecked != true)
-            return false;
-
-        var query = _nativeFatSearchBox?.Text?.Trim();
-        if (string.IsNullOrWhiteSpace(query))
-            return true;
-
-        return row.SignalName.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.IecReference.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.IecTelegram.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.DataType.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.LiveValue.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.Value1Text.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.Value2Text.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.Result.Contains(query, StringComparison.OrdinalIgnoreCase) ||
-               row.StatusText.Contains(query, StringComparison.OrdinalIgnoreCase);
-    }
-
-    private void NativeFatSearchBox_TextChanged(object sender, TextChangedEventArgs e)
-        => _nativeFatView?.Refresh();
-
-    private void NativeFatShowHistorical_Changed(object sender, RoutedEventArgs e)
-    {
-        _nativeFatView?.Refresh();
-        UpdateNativeFatSummary();
-    }
-
-    private void NativeFatRow_StateChanged(object? sender, EventArgs e)
-    {
-        QueueNativeFatSave();
-        UpdateNativeFatSummary();
-        _nativeFatPreviewGrid?.Items.Refresh();
-    }
-
-    private IReadOnlyList<NativeFatSignalRow> SelectedNativeFatRows()
-    {
-        if (_nativeFatGrid == null)
-            return Array.Empty<NativeFatSignalRow>();
-
-        var selected = _nativeFatGrid.SelectedItems.OfType<NativeFatSignalRow>().ToArray();
-        if (selected.Length > 0)
-            return selected;
-        return _nativeFatGrid.CurrentItem is NativeFatSignalRow current
-            ? new[] { current }
-            : Array.Empty<NativeFatSignalRow>();
-    }
-
-    private void NativeFatRefresh_Click(object sender, RoutedEventArgs e)
-        => _ = EnsureNativeFatLoadedAsync(forceReconcile: true);
-
-    private void NativeFatCapture1_Click(object sender, RoutedEventArgs e)
-        => CaptureNativeFatSelection(1);
-
-    private void NativeFatCapture2_Click(object sender, RoutedEventArgs e)
-        => CaptureNativeFatSelection(2);
-
-    private void CaptureNativeFatSelection(int slot)
-    {
-        var rows = SelectedNativeFatRows();
-        if (rows.Count == 0)
-        {
-            SetNativeFatStatus("Select one or more FAT signals first.");
-            return;
-        }
-
-        var captured = 0;
-        foreach (var row in rows)
-        {
-            if (row.CaptureValue(slot))
-                captured++;
-        }
-        SetNativeFatStatus(captured == 0
-            ? "No selected row has a current Explorer signal source. Historical rows cannot be captured."
-            : $"Captured Value {slot} for {captured} signal(s). Autosave queued.");
-    }
-
-    private void NativeFatPass_Click(object sender, RoutedEventArgs e)
-        => SetNativeFatSelectionResult(NativeFatResult.Pass);
-
-    private void NativeFatReview_Click(object sender, RoutedEventArgs e)
-        => SetNativeFatSelectionResult(NativeFatResult.Review);
-
-    private void NativeFatFail_Click(object sender, RoutedEventArgs e)
-        => SetNativeFatSelectionResult(NativeFatResult.Fail);
-
-    private void SetNativeFatSelectionResult(string result)
-    {
-        var rows = SelectedNativeFatRows();
-        if (rows.Count == 0)
-        {
-            SetNativeFatStatus("Select one or more FAT signals first.");
-            return;
-        }
-
-        foreach (var row in rows)
-            row.SetResult(result);
-        SetNativeFatStatus($"{rows.Count} signal(s) marked {result}. Previous state retained in history.");
-    }
-
-    private void NativeFatReset_Click(object sender, RoutedEventArgs e)
-    {
-        var rows = SelectedNativeFatRows();
-        if (rows.Count == 0)
-        {
-            SetNativeFatStatus("Select one or more FAT signals first.");
-            return;
-        }
-
-        foreach (var row in rows)
-            row.ResetCurrentResult();
-        SetNativeFatStatus($"Reset current FAT state for {rows.Count} signal(s); history was retained.");
-    }
-
-    private void NativeFatReportPreview_Click(object sender, RoutedEventArgs e)
-    {
-        _nativeFatPreviewVisible = !_nativeFatPreviewVisible;
-        if (_nativeFatPreviewPane == null || _nativeFatPreviewGapColumn == null || _nativeFatPreviewColumn == null)
-            return;
-
-        _nativeFatPreviewPane.Visibility = _nativeFatPreviewVisible ? Visibility.Visible : Visibility.Collapsed;
-        _nativeFatPreviewGapColumn.Width = new GridLength(_nativeFatPreviewVisible ? 10 : 0);
-        _nativeFatPreviewColumn.Width = _nativeFatPreviewVisible ? new GridLength(330) : new GridLength(0);
-        UpdateNativeFatSummary();
-    }
-
-    private void QueueNativeFatSave()
-    {
-        if (_nativeFatCurrentState == null || _nativeFatSaveTimer == null)
-            return;
-        _nativeFatSaveTimer.Stop();
-        _nativeFatSaveTimer.Start();
-    }
-
-    private async void NativeFatSaveTimer_Tick(object? sender, EventArgs e)
-    {
-        _nativeFatSaveTimer?.Stop();
-        if (_nativeFatCurrentState != null)
-            await SaveNativeFatStateAsync(_nativeFatCurrentState);
-    }
-
-    private async Task SaveNativeFatStateAsync(NativeFatDeviceState state)
-    {
-        try
-        {
-            await _nativeFatSaveGate.WaitAsync(_applicationCancellation.Token);
-            try
-            {
-                await _nativeFatStore.SaveAsync(state, _applicationCancellation.Token);
-            }
-            finally
-            {
-                _nativeFatSaveGate.Release();
-            }
-
-            if (ReferenceEquals(state, _nativeFatCurrentState))
-            {
-                SetNativeFatStatus($"Saved · {DateTime.Now:HH:mm:ss}");
-                UpdateNativeFatSummary();
-            }
-        }
-        catch (OperationCanceledException)
-        {
-            // Application shutdown or superseded state.
-        }
-        catch (Exception ex)
-        {
-            AddLog("WARN", "Native FAT", $"Autosave failed: {ex.Message}");
-            if (ReferenceEquals(state, _nativeFatCurrentState))
-                SetNativeFatStatus("Autosave failed; current in-memory FAT state is still available.");
-        }
-    }
-
-    private void UpdateNativeFatSummary()
-    {
-        var device = SelectedDevice;
-        var deviceLabel = device == null ? "NONE" : device.Name;
-        if (_nativeFatContextText != null)
-            _nativeFatContextText.Text = $"IED · {deviceLabel}";
-        if (_nativeFatPreviewDeviceText != null)
-            _nativeFatPreviewDeviceText.Text = $"IED · {deviceLabel}";
-
-        var current = _nativeFatRows.Count(row => !row.IsHistorical);
-        var historical = _nativeFatRows.Count(row => row.IsHistorical);
-        var pass = _nativeFatRows.Count(row => !row.IsHistorical && row.Result == NativeFatResult.Pass);
-        var review = _nativeFatRows.Count(row => !row.IsHistorical && row.Result == NativeFatResult.Review);
-        var fail = _nativeFatRows.Count(row => !row.IsHistorical && row.Result == NativeFatResult.Fail);
-        var untested = Math.Max(0, current - pass - review - fail);
-
-        if (_nativeFatPreviewSummaryText != null)
-        {
-            _nativeFatPreviewSummaryText.Text = device == null
-                ? "Select an IED in the persistent Explorer."
-                : $"Current {current} · PASS {pass} · REVIEW {review} · FAIL {fail} · UNTESTED {untested} · historical {historical}";
-        }
-
-        if (_nativeFatPersistenceText != null)
-        {
-            _nativeFatPersistenceText.Text = _nativeFatCurrentState == null
-                ? "Per-IED JSON · non-destructive reconciliation"
-                : $"Resume file: {Path.GetFileName(_nativeFatCurrentState.StoragePath)}\nHistorical IEC identities remain stored when engineering changes.";
-        }
-
-        if (!_nativeFatLoading && device != null && _nativeFatStatusText != null &&
-            (_nativeFatStatusText.Text.StartsWith("Select", StringComparison.OrdinalIgnoreCase) ||
-             _nativeFatStatusText.Text.StartsWith("Loading", StringComparison.OrdinalIgnoreCase)))
-        {
-            _nativeFatStatusText.Text = $"{current} current · {historical} historical · {untested} untested";
-        }
-    }
-
-    private void SetNativeFatStatus(string text)
-    {
-        if (_nativeFatStatusText != null)
-            _nativeFatStatusText.Text = text;
-    }
-
-    private static string StableNativeFatDeviceKey(Iec61850MonitorDevice device)
-        => !string.IsNullOrWhiteSpace(device.DeviceId) ? device.DeviceId : device.Name;
-
-    private Brush ResourceBrush(string key, Color fallback)
-        => TryFindResource(key) as Brush ?? new SolidColorBrush(fallback);
-
-    private void NativeFat_MainWindowClosed(object? sender, EventArgs e)
-    {
-        _nativeFatInstallRetry?.Stop();
-        _nativeFatReconcileTimer?.Stop();
-        _nativeFatSaveTimer?.Stop();
-        _nativeFatLoadCts?.Cancel();
-        _nativeFatLoadCts?.Dispose();
-
-        PropertyChanged -= NativeFat_MainWindowPropertyChanged;
-        MainTabs.SelectionChanged -= NativeFat_MainTabsSelectionChanged;
-        SizeChanged -= NativeFat_MainWindowSizeChanged;
-        Closed -= NativeFat_MainWindowClosed;
-
-        if (_nativeFatObservedDevice != null)
-        {
-            _nativeFatObservedDevice.Signals.CollectionChanged -= NativeFatSignalCollectionChanged;
-            _nativeFatObservedDevice.Points.CollectionChanged -= NativeFatSignalCollectionChanged;
-        }
-
-        ClearNativeFatRows();
-        _nativeFatSaveGate.Dispose();
+            new Action(() => UpdateNavigationVisuals(MainTabs.SelectedIndex, animate: false)));
     }
 }
diff --git a/MainWindow.NavigationLayoutFix.cs b/MainWindow.NavigationLayoutFix.cs
index fe2e8325f..55e5b0f0b 100644
--- a/MainWindow.NavigationLayoutFix.cs
+++ b/MainWindow.NavigationLayoutFix.cs
@@ -10,7 +10,7 @@ namespace ArIED61850Tester;
 /// <summary>
 /// Owns the responsive geometry of the MainWindow workflow header.
 ///
-/// The original XAML used a 760 px shell split into six equal columns while the
+/// The original XAML used a 760 px shell split into seven equal columns while the
 /// selection pill moved in hard-coded 150 px steps. That was barely large enough for
 /// short labels and clipped "IEC 61850 Explorer" / "GOOSE Subscriber" once the center
 /// workspace switch and live connection/status chips were also present. This behavior
@@ -93,7 +93,7 @@ private static void OnMainWindowButtonClick(object sender, RoutedEventArgs e)
         if (sender is not MainWindow window || e.Source is not Button button)
             return;
 
-        if (button.Name is not ("NavExplorerButton" or "NavLiveButton" or "NavEventsButton" or "NavAlarmButton" or "NavGooseButton" or "NavDiagnosticsButton"))
+        if (button.Name is not ("NavExplorerButton" or "NavLiveButton" or "NavEventsButton" or "NavAlarmButton" or "NavGooseButton" or "NavDiagnosticsButton" or "NavNativeFatButton"))
             return;
 
         // A repeated click on the already-selected tab does not raise SelectionChanged,
@@ -195,7 +195,8 @@ private static void ApplyResponsiveLayout(MainWindow window)
             window.FindName("NavEventsButton") as Button,
             window.FindName("NavAlarmButton") as Button,
             window.FindName("NavGooseButton") as Button,
-            window.FindName("NavDiagnosticsButton") as Button
+            window.FindName("NavDiagnosticsButton") as Button,
+            window.FindName("NavNativeFatButton") as Button
         ];
 
     private static void UpdatePillGeometry(MainWindow window, double shellWidth)
@@ -204,9 +205,9 @@ private static void UpdatePillGeometry(MainWindow window, double shellWidth)
             return;
 
         // Border padding owns 10 px horizontally. The nav grid itself is divided into
-        // six equal star columns, so this is the exact width used by each button cell.
+        // seven equal star columns, so this is the exact width used by each button cell.
         var contentWidth = Math.Max(0d, shellWidth - 10d);
-        var cellWidth = contentWidth / 6d;
+        var cellWidth = contentWidth / 7d;
         pill.Width = Math.Max(1d, cellWidth - 2d);
         pill.Height = 36;
         pill.HorizontalAlignment = HorizontalAlignment.Left;
@@ -279,8 +280,8 @@ private static void PositionPill(MainWindow window, bool animate)
         if (contentWidth <= 0d)
             contentWidth = Math.Max(0d, shell.Width - shell.Padding.Left - shell.Padding.Right);
 
-        var cellWidth = contentWidth / 6d;
-        var target = Math.Clamp(tabs.SelectedIndex, 0, 5) * cellWidth;
+        var cellWidth = contentWidth / 7d;
+        var target = Math.Clamp(tabs.SelectedIndex, 0, 6) * cellWidth;
         pill.Width = Math.Max(1d, cellWidth - 2d);
 
         translate.BeginAnimation(TranslateTransform.XProperty, null);
diff --git a/MainWindow.ProductionFatEngineeringBootstrap.cs b/MainWindow.ProductionFatEngineeringBootstrap.cs
index f7c8dfb36..28ef4686d 100644
--- a/MainWindow.ProductionFatEngineeringBootstrap.cs
+++ b/MainWindow.ProductionFatEngineeringBootstrap.cs
@@ -38,7 +38,6 @@ private static void ProductionFatEngineeringBootstrap_Loaded(object sender, Rout
         window.MainTabs.SelectionChanged += window.ProductionFatEngineeringBootstrap_SelectionChanged;
         window.PropertyChanged += window.ProductionFatEngineeringBootstrap_PropertyChanged;
         window.Closed += window.ProductionFatEngineeringBootstrap_Closed;
-        window.QueueProductionFatEngineeringBootstrap();
     }
 
     private void ProductionFatEngineeringBootstrap_SelectionChanged(object sender, SelectionChangedEventArgs e)
@@ -55,13 +54,16 @@ private void ProductionFatEngineeringBootstrap_PropertyChanged(object? sender, P
 
         // A running production FAT session owns its latched IED. The normal embedded-host
         // selection bridge handles idle retargeting. Bootstrap is only needed before a FAT
-        // workspace exists.
+        // workspace exists and only while the operator is actually entering FAT.
         if (_productionFatWindow == null && _loadedIoFatWindow == null)
             QueueProductionFatEngineeringBootstrap();
     }
 
     private void QueueProductionFatEngineeringBootstrap()
     {
+        // FAT preparation must never run in the background while Engineering is connecting
+        // or monitoring. The shared Engineering acquisition session remains authoritative;
+        // clicking FAT is the only navigation event allowed to build the FAT projection.
         if (!_productionFatEngineeringBootstrapInstalled || MainTabs.SelectedIndex != NativeFatWorkspaceIndex)
             return;
 
@@ -89,6 +91,10 @@ private async Task EnsureProductionFatFromEngineeringAsync()
         if (selected?.SclWorkspace == null ||
             selected.SclWorkspace.DesignModel.DataSets.Sum(dataSet => dataSet.Members.Count) == 0)
         {
+            var message = selected == null
+                ? "Select an Engineering IED with a static DataSet to prepare FAT."
+                : $"{selected.Name} has no static DataSet scope in the Engineering SCL model.";
+            ShowProductionFatBootstrapState(message, isBusy: false);
             SetStatus(selected == null
                 ? "FAT · select an Engineering IED with a static DataSet."
                 : $"FAT · {selected.Name} has no static DataSet scope in the Engineering SCL model.");
@@ -102,6 +108,8 @@ private async Task EnsureProductionFatFromEngineeringAsync()
             .ToArray();
         if (engineeringDevices.All(device => !ReferenceEquals(device, selected)))
         {
+            var message = $"Engineering source provenance for {selected.Name} is unavailable. Reopen the SCL source to restore FAT authority.";
+            ShowProductionFatBootstrapState(message, isBusy: false);
             SetStatus($"FAT · Engineering source provenance for {selected.Name} is unavailable; use Open SCL to restore the source authority.");
             return;
         }
@@ -111,6 +119,9 @@ private async Task EnsureProductionFatFromEngineeringAsync()
         _productionFatEngineeringBootstrapCts?.Dispose();
         _productionFatEngineeringBootstrapCts = CancellationTokenSource.CreateLinkedTokenSource(_applicationCancellation.Token);
         var token = _productionFatEngineeringBootstrapCts.Token;
+        ShowProductionFatBootstrapState(
+            $"Reusing {selected.Name} from the Engineering static DataSet authority. No reconnect or SCL re-import is started.",
+            isBusy: true);
         SetStatus($"FAT · preparing {selected.Name} from the Engineering static DataSet…");
 
         try
@@ -125,9 +136,12 @@ private async Task EnsureProductionFatFromEngineeringAsync()
             // reparsing XML or starting a second model/acquisition stack.
             _ioFatSclProjectImportService.AdoptEngineeringRuntimeWorkspaces(projection.RuntimeWorkspaces);
 
-            var launch = await IoTestWorkspaceBootstrapService.OpenSourcesAsync(
+            // Projection already SHA-256-described the canonical Engineering SCL source set.
+            // Carry those exact identities through bootstrap/persistence instead of hashing
+            // the same files again. Staging still verifies every copied byte against SHA-256.
+            var launch = await IoTestWorkspaceBootstrapService.OpenDescribedSourcesAsync(
                 projection.Project,
-                projection.SourceInputs,
+                projection.DescribedSources,
                 IoTestingProjectsRoot(),
                 IoTestingEvidenceRoot(),
                 CreateIoTestSession,
@@ -159,7 +173,7 @@ private async Task EnsureProductionFatFromEngineeringAsync()
                              ?? ResolveIoTestDevice(ied.IpAddress)
                              ?? ResolveIoTestDevice(ied.IedName);
                 if (device is not null)
-                    MarkSharedSelectionAuthority(device);
+                    PreserveSharedStaticDataSetAuthority(device);
             }
 
             launch.Workspace.ScheduleSave();
@@ -170,10 +184,19 @@ private async Task EnsureProductionFatFromEngineeringAsync()
         catch (OperationCanceledException)
         {
             // Fast navigation/close is normal. No modal interruption is appropriate here.
+            if (MainTabs.SelectedIndex == NativeFatWorkspaceIndex && _productionFatWindow == null)
+            {
+                ShowProductionFatBootstrapState(
+                    "FAT preparation was cancelled. Select the FAT tab again to retry from the current Engineering authority.",
+                    isBusy: false);
+            }
         }
         catch (Exception ex) when (ex is IOException or JsonException or InvalidDataException or UnauthorizedAccessException or ArgumentException or InvalidOperationException)
         {
             AddLog("WARN", "FAT", $"Automatic Engineering FAT bootstrap unavailable: {ex.Message}");
+            ShowProductionFatBootstrapState(
+                $"FAT could not reuse the current Engineering static DataSet: {ex.Message}",
+                isBusy: false);
             SetStatus($"FAT · could not reuse the Engineering static DataSet automatically: {ex.Message}");
         }
         finally
diff --git a/MainWindow.ProductionFatFieldPerformance.cs b/MainWindow.ProductionFatFieldPerformance.cs
new file mode 100644
index 000000000..ffbfa80ab
--- /dev/null
+++ b/MainWindow.ProductionFatFieldPerformance.cs
@@ -0,0 +1,75 @@
+using System.Runtime.CompilerServices;
+using System.Windows;
+using System.Windows.Threading;
+
+namespace ArIED61850Tester;
+
+/// <summary>
+/// Field guard for the embedded Static DataSet FAT path.
+/// The legacy standalone FAT workspace temporarily tightened MainWindow polling to 250 ms.
+/// That is unnecessary when FAT is consuming the already-running Engineering Static RCB/URCB
+/// report-only session, and keeping it active after embedding adds avoidable UI/runtime load
+/// while the operator navigates to other Engineering tabs.
+/// </summary>
+public partial class MainWindow
+{
+    [ModuleInitializer]
+    internal static void RegisterProductionFatFieldPerformanceGuard()
+    {
+        EventManager.RegisterClassHandler(
+            typeof(IoListTestingWindow),
+            FrameworkElement.LoadedEvent,
+            new RoutedEventHandler(ProductionFatFieldPerformance_IoWindowLoaded),
+            handledEventsToo: true);
+    }
+
+    private static void ProductionFatFieldPerformance_IoWindowLoaded(object sender, RoutedEventArgs e)
+    {
+        if (sender is not IoListTestingWindow window ||
+            !ReferenceEquals(e.OriginalSource, window) ||
+            window.Owner is not MainWindow owner ||
+            !owner.ProductionFatTabReady)
+        {
+            return;
+        }
+
+        // EmbeddedEngineeringHost mounts at Loaded priority. Run after that short operation,
+        // then release only the legacy cyclic-polling acceleration. FAT evidence continues to
+        // arrive from the exact shared Engineering report session and Runtime.PointUpdated.
+        owner.Dispatcher.BeginInvoke(
+            DispatcherPriority.ContextIdle,
+            new Action(() => owner.ReleaseLegacyPollingOverrideForStaticEmbeddedFat(window)));
+    }
+
+    private void ReleaseLegacyPollingOverrideForStaticEmbeddedFat(IoListTestingWindow window)
+    {
+        if (!window.IsLoaded ||
+            !ReferenceEquals(_loadedIoFatWindow, window) ||
+            !_pollingIntervalBeforeIoFat.HasValue)
+        {
+            return;
+        }
+
+        var liveDevices = window.Project.Ieds
+            .Select(ied => ResolveIoTestDevice(ied.LiveDeviceId)
+                           ?? ResolveIoTestDevice(ied.IpAddress)
+                           ?? ResolveIoTestDevice(ied.IedName))
+            .Where(device => device is not null)
+            .Distinct()
+            .ToArray();
+
+        // Workbook/manual FAT can still legitimately use the legacy faster polling cadence.
+        // Automatic Engineering FAT must not: every participating live IED must already own
+        // Static DataSet report-only authority before this override is released.
+        if (liveDevices.Length == 0 || liveDevices.Any(device => !IsSharedStaticDataSetAuthority(device!)))
+            return;
+
+        PollingIntervalMs = _pollingIntervalBeforeIoFat.Value;
+        _pollingIntervalBeforeIoFat = null;
+
+        AddLog(
+            "INFO",
+            "FAT",
+            $"Embedded Static DataSet FAT preserved Engineering polling cadence ({PollingIntervalMs} ms); no cyclic MMS acceleration remains active.");
+    }
+}
diff --git a/MainWindow.ProductionFatNavigationParity.cs b/MainWindow.ProductionFatNavigationParity.cs
deleted file mode 100644
index 61787678d..000000000
--- a/MainWindow.ProductionFatNavigationParity.cs
+++ /dev/null
@@ -1,173 +0,0 @@
-using System.Runtime.CompilerServices;
-using System.Windows;
-using System.Windows.Controls;
-using System.Windows.Media;
-using System.Windows.Media.Animation;
-using System.Windows.Threading;
-
-namespace ArIED61850Tester;
-
-/// <summary>
-/// Extends the existing six-destination responsive nav contract to the dynamically-added
-/// production FAT destination. The legacy MainWindow navigation code clamps index 6 to 5;
-/// this late correction keeps Diagnostics from remaining highlighted while FAT is active
-/// and moves the same selection pill into the seventh equal-width cell.
-/// </summary>
-internal static class MainWindowProductionFatNavigationParity
-{
-    private static readonly string[] NavigationButtonNames =
-    [
-        "NavExplorerButton",
-        "NavLiveButton",
-        "NavEventsButton",
-        "NavAlarmButton",
-        "NavGooseButton",
-        "NavDiagnosticsButton",
-        "NavNativeFatButton"
-    ];
-
-    [ModuleInitializer]
-    internal static void Register()
-    {
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            FrameworkElement.LoadedEvent,
-            new RoutedEventHandler(OnLoaded),
-            handledEventsToo: true);
-        EventManager.RegisterClassHandler(
-            typeof(MainWindow),
-            Button.ClickEvent,
-            new RoutedEventHandler(OnButtonClick),
-            handledEventsToo: true);
-    }
-
-    private static void OnLoaded(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window)
-            return;
-
-        window.SizeChanged -= Window_SizeChanged;
-        window.SizeChanged += Window_SizeChanged;
-        if (window.FindName("MainTabs") is TabControl tabs)
-        {
-            tabs.SelectionChanged -= Tabs_SelectionChanged;
-            tabs.SelectionChanged += Tabs_SelectionChanged;
-        }
-        Queue(window, animate: false);
-    }
-
-    private static void OnButtonClick(object sender, RoutedEventArgs e)
-    {
-        if (sender is not MainWindow window || e.Source is not Button button ||
-            !NavigationButtonNames.Contains(button.Name, StringComparer.Ordinal))
-        {
-            return;
-        }
-        Queue(window, animate: true);
-    }
-
-    private static void Tabs_SelectionChanged(object sender, SelectionChangedEventArgs e)
-    {
-        if (sender is not TabControl tabs || !ReferenceEquals(e.Source, tabs) ||
-            Window.GetWindow(tabs) is not MainWindow window)
-        {
-            return;
-        }
-        Queue(window, animate: true);
-    }
-
-    private static void Window_SizeChanged(object sender, SizeChangedEventArgs e)
-    {
-        if (sender is MainWindow window)
-            Queue(window, animate: false);
-    }
-
-    private static void Queue(MainWindow window, bool animate)
-    {
-        // Existing MainWindow + responsive-layout handlers still perform their historical
-        // six-slot correction. ApplicationIdle deliberately runs after those handlers so
-        // the seven-slot Engineering shell is the final visual authority.
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(() => Apply(window, animate)));
-    }
-
-    private static void Apply(MainWindow window, bool animate)
-    {
-        if (window.FindName("MainTabs") is not TabControl tabs || tabs.Items.Count < 7 ||
-            window.FindName("WorkflowNavGrid") is not Grid grid ||
-            window.FindName("WorkflowNavShell") is not Border shell ||
-            window.FindName("WorkflowPill") is not Border pill)
-        {
-            return;
-        }
-
-        var fatButton = grid.Children
-            .OfType<Button>()
-            .FirstOrDefault(button => button.Name.Equals("NavNativeFatButton", StringComparison.Ordinal));
-        var buttons = new Button?[]
-        {
-            window.FindName("NavExplorerButton") as Button,
-            window.FindName("NavLiveButton") as Button,
-            window.FindName("NavEventsButton") as Button,
-            window.FindName("NavAlarmButton") as Button,
-            window.FindName("NavGooseButton") as Button,
-            window.FindName("NavDiagnosticsButton") as Button,
-            fatButton
-        };
-        if (buttons.Any(button => button == null))
-            return;
-
-        while (grid.ColumnDefinitions.Count < 7)
-            grid.ColumnDefinitions.Add(new ColumnDefinition());
-        foreach (var column in grid.ColumnDefinitions.Take(7))
-            column.Width = new GridLength(1, GridUnitType.Star);
-
-        // FAT must use the exact same button style and density as its siblings.
-        var reference = buttons[5]!;
-        var muted = window.TryFindResource("Muted") as Brush ?? new SolidColorBrush(Color.FromRgb(0x5F, 0x6B, 0x7A));
-        var selectedIndex = Math.Clamp(tabs.SelectedIndex, 0, 6);
-        for (var index = 0; index < buttons.Length; index++)
-        {
-            var button = buttons[index]!;
-            button.Style ??= reference.Style;
-            button.MinHeight = reference.MinHeight;
-            button.MinWidth = 0;
-            button.Margin = reference.Margin;
-            button.Padding = reference.Padding;
-            button.HorizontalContentAlignment = reference.HorizontalContentAlignment;
-            button.VerticalAlignment = reference.VerticalAlignment;
-            button.VerticalContentAlignment = reference.VerticalContentAlignment;
-            button.Foreground = index == selectedIndex ? Brushes.White : muted;
-        }
-
-        var contentWidth = Math.Max(0d, shell.ActualWidth - shell.Padding.Left - shell.Padding.Right);
-        if (contentWidth <= 0d)
-            contentWidth = Math.Max(0d, shell.Width - shell.Padding.Left - shell.Padding.Right);
-        var cellWidth = contentWidth / 7d;
-        pill.Width = Math.Max(1d, cellWidth - 2d);
-        pill.Height = 36;
-
-        var translate = window.FindName("WorkflowPillTranslate") as TranslateTransform;
-        if (translate == null && pill.RenderTransform is TransformGroup group)
-            translate = group.Children.OfType<TranslateTransform>().LastOrDefault();
-        if (translate == null)
-            return;
-
-        var target = selectedIndex * cellWidth;
-        translate.BeginAnimation(TranslateTransform.XProperty, null);
-        if (!animate)
-        {
-            translate.X = target;
-            return;
-        }
-
-        translate.BeginAnimation(
-            TranslateTransform.XProperty,
-            new DoubleAnimation(target, TimeSpan.FromMilliseconds(190))
-            {
-                EasingFunction = new CubicEase { EasingMode = EasingMode.EaseOut },
-                FillBehavior = FillBehavior.HoldEnd
-            });
-    }
-}
diff --git a/MainWindow.ProductionFatNoFlicker.cs b/MainWindow.ProductionFatNoFlicker.cs
index e976ca883..f346e68f2 100644
--- a/MainWindow.ProductionFatNoFlicker.cs
+++ b/MainWindow.ProductionFatNoFlicker.cs
@@ -24,6 +24,5 @@ public partial class MainWindow
 
     private bool ShouldKeepEngineeringVisibleDuringProductionFatBootstrap()
         => _productionFatEngineeringBootstrapBusy &&
-           ProductionFatTabReady &&
-           MainTabs.SelectedIndex == NativeFatWorkspaceIndex;
+           ProductionFatTabReady;
 }
diff --git a/MainWindow.ProductionFatTab.cs b/MainWindow.ProductionFatTab.cs
index b0d01d9e3..f09a3ae39 100644
--- a/MainWindow.ProductionFatTab.cs
+++ b/MainWindow.ProductionFatTab.cs
@@ -8,7 +8,7 @@
 namespace ArIED61850Tester;
 
 /// <summary>
-/// P2 FAT pivot: the Engineering FAT tab is a host for the proven production
+/// Engineering FAT pivot: the permanent XAML FAT tab hosts the proven production
 /// IoListTestingWindow workspace rather than a second/manual FAT implementation.
 /// The global Engineering IED Explorer and shared Command Dock remain authoritative.
 /// </summary>
@@ -19,7 +19,7 @@ public partial class MainWindow
     private FrameworkElement? _productionFatSurface;
     private DispatcherTimer? _productionFatInstallRetry;
 
-    internal bool ProductionFatTabReady => _productionFatTabInstalled && _nativeFatTab != null;
+    internal bool ProductionFatTabReady => _productionFatTabInstalled && NativeFatTab != null;
 
     [ModuleInitializer]
     internal static void RegisterProductionFatTabPivot()
@@ -36,9 +36,10 @@ private static void ProductionFatTab_MainWindowLoaded(object sender, RoutedEvent
         if (sender is not MainWindow window || window._productionFatTabInstalled)
             return;
 
-        window.Dispatcher.BeginInvoke(
-            DispatcherPriority.ApplicationIdle,
-            new Action(window.TryInstallProductionFatTabPivot));
+        // MainWindow.Loaded runs after XAML has materialized the canonical seventh tab but
+        // before the first normal render. Install the permanent FAT shell immediately so the
+        // operator never sees an empty/black seventh workspace while idle-dispatcher work waits.
+        window.TryInstallProductionFatTabPivot();
     }
 
     private void TryInstallProductionFatTabPivot()
@@ -46,13 +47,14 @@ private void TryInstallProductionFatTabPivot()
         if (_productionFatTabInstalled || !IsLoaded)
             return;
 
-        // PR #290 creates the seventh navigation slot. Wait until that shell contribution
-        // exists, then replace only its FAT content/handlers; P0/P1 shell geometry is untouched.
-        if (!_nativeFatInstalled || _nativeFatTab == null || _nativeFatNavButton == null)
+        // M7: MainWindow.xaml is the sole owner of the seventh destination. Wait only
+        // until the canonical XAML tab is present; there is no native FAT runtime to install.
+        if (MainTabs.Items.Count <= NativeFatWorkspaceIndex ||
+            !ReferenceEquals(MainTabs.Items[NativeFatWorkspaceIndex], NativeFatTab))
         {
-            _productionFatInstallRetry ??= new DispatcherTimer(DispatcherPriority.ApplicationIdle)
+            _productionFatInstallRetry ??= new DispatcherTimer(DispatcherPriority.Loaded)
             {
-                Interval = TimeSpan.FromMilliseconds(120)
+                Interval = TimeSpan.FromMilliseconds(60)
             };
             _productionFatInstallRetry.Tick -= ProductionFatInstallRetry_Tick;
             _productionFatInstallRetry.Tick += ProductionFatInstallRetry_Tick;
@@ -62,26 +64,14 @@ private void TryInstallProductionFatTabPivot()
 
         _productionFatInstallRetry?.Stop();
         _productionFatTabInstalled = true;
+        NativeFatTab.Content = BuildProductionFatPermanentHost();
 
-        // Retire the experimental manual-capture surface. Keep its code isolated in the
-        // branch for now so this pivot is reversible while production FAT parity is verified.
-        _nativeFatReconcileTimer?.Stop();
-        _nativeFatSaveTimer?.Stop();
-        AttachNativeFatObservedDevice(null);
-        PropertyChanged -= NativeFat_MainWindowPropertyChanged;
-        MainTabs.SelectionChanged -= NativeFat_MainTabsSelectionChanged;
-        _nativeFatNavButton.Click -= NativeFatNavButton_Click;
-
-        _nativeFatTab.Content = BuildProductionFatLauncher();
-
-        // FAT must be visually indistinguishable from the existing workflow tabs.
-        _nativeFatNavButton.Style = NavDiagnosticsButton.Style;
-        _nativeFatNavButton.Padding = NavDiagnosticsButton.Padding;
-        _nativeFatNavButton.Margin = NavDiagnosticsButton.Margin;
-        _nativeFatNavButton.HorizontalContentAlignment = NavDiagnosticsButton.HorizontalContentAlignment;
-        _nativeFatNavButton.VerticalContentAlignment = NavDiagnosticsButton.VerticalContentAlignment;
-        _nativeFatNavButton.ToolTip = "Production FAT workspace · automatic Value 1 / Value 2 evidence capture";
-        _nativeFatNavButton.Click += ProductionFatNavButton_Click;
+        // Bootstrap remains navigation-gated. Queueing here is harmless because the
+        // Engineering bootstrap itself refuses to build FAT unless the FAT tab is active.
+        QueueProductionFatEngineeringBootstrap();
+
+        // MainWindow.xaml owns both style and click routing for the seventh nav button.
+        NavNativeFatButton.ToolTip = "Production FAT workspace · automatic Value 1 / Value 2 evidence capture";
 
         PropertyChanged += ProductionFat_MainWindowPropertyChanged;
         MainTabs.SelectionChanged += ProductionFat_MainTabsSelectionChanged;
@@ -96,97 +86,79 @@ private void ProductionFatInstallRetry_Tick(object? sender, EventArgs e)
         TryInstallProductionFatTabPivot();
     }
 
-    private FrameworkElement BuildProductionFatLauncher()
+    private FrameworkElement BuildProductionFatPermanentHost(
+        string? statusText = null,
+        bool isBusy = false)
     {
         var root = new Grid { Margin = new Thickness(0) };
         var card = new Border
         {
-            MaxWidth = 720,
-            Padding = new Thickness(28, 24, 28, 24),
-            CornerRadius = new CornerRadius(16),
-            Background = TryFindResource("Surface") as Brush ?? Brushes.White,
-            BorderBrush = TryFindResource("Line") as Brush ?? new SolidColorBrush(Color.FromRgb(0xDC, 0xE4, 0xEF)),
+            MaxWidth = 520,
+            Padding = new Thickness(28, 24, 28, 22),
+            CornerRadius = new CornerRadius(18),
+            Background = TryFindResource("CardBackground") as Brush ?? Brushes.White,
+            BorderBrush = TryFindResource("CardBorder") as Brush ?? new SolidColorBrush(Color.FromRgb(220, 228, 239)),
             BorderThickness = new Thickness(1),
             HorizontalAlignment = HorizontalAlignment.Center,
             VerticalAlignment = VerticalAlignment.Center
         };
-
-        var content = new StackPanel();
-        content.Children.Add(new TextBlock
+        var panel = new StackPanel
+        {
+            HorizontalAlignment = HorizontalAlignment.Stretch
+        };
+        panel.Children.Add(new TextBlock
         {
-            Text = "FAT WORKSPACE",
-            Style = TryFindResource("MicroLabel") as Style,
-            Foreground = TryFindResource("Accent") as Brush,
+            Text = "IEC 61850 FAT",
+            FontSize = 11,
+            FontWeight = FontWeights.SemiBold,
+            Foreground = TryFindResource("Accent") as Brush ?? Brushes.RoyalBlue,
             HorizontalAlignment = HorizontalAlignment.Center
         });
-        content.Children.Add(new TextBlock
+        panel.Children.Add(new TextBlock
         {
-            Text = "Production FAT inside Engineering",
-            Margin = new Thickness(0, 7, 0, 0),
-            FontSize = 22,
+            Text = isBusy ? "Preparing production workspace" : "Production FAT workspace",
+            FontSize = 18,
             FontWeight = FontWeights.SemiBold,
             Foreground = TryFindResource("Ink") as Brush ?? Brushes.Black,
+            Margin = new Thickness(0, 5, 0, 0),
             HorizontalAlignment = HorizontalAlignment.Center
         });
-        content.Children.Add(new TextBlock
+        panel.Children.Add(new TextBlock
         {
-            Text = "Open an SCL FAT project to use the proven automatic capture / completion workflow. The global IED Explorer stays visible at left and the shared Command Dock remains the only command surface.",
-            Margin = new Thickness(0, 10, 0, 18),
-            MaxWidth = 610,
+            Text = string.IsNullOrWhiteSpace(statusText)
+                ? "Select an Engineering IED with static DataSet scope, then open FAT."
+                : statusText,
+            FontSize = 11.5,
+            Foreground = TryFindResource("Muted") as Brush ?? Brushes.DimGray,
             TextAlignment = TextAlignment.Center,
             TextWrapping = TextWrapping.Wrap,
-            FontSize = 12.2,
-            Foreground = TryFindResource("Muted") as Brush ?? Brushes.DimGray
+            Margin = new Thickness(0, 8, 0, 0)
         });
 
-        var actions = new StackPanel
-        {
-            Orientation = Orientation.Horizontal,
-            HorizontalAlignment = HorizontalAlignment.Center
-        };
-        var openScl = new Button
-        {
-            Content = "Open SCL for FAT",
-            Style = TryFindResource("PrimaryButton") as Style,
-            Padding = new Thickness(14, 8, 14, 8),
-            Margin = new Thickness(0, 0, 8, 0)
-        };
-        openScl.Click += OpenSclFatTesting_Click;
-        actions.Children.Add(openScl);
-
-        var openProject = new Button
+        if (isBusy)
         {
-            Content = "Open ARSAS Project",
-            Style = TryFindResource("SoftButton") as Style,
-            Padding = new Thickness(14, 8, 14, 8)
-        };
-        openProject.Click += OpenIoListPackage_Click;
-        actions.Children.Add(openProject);
-        content.Children.Add(actions);
-
-        content.Children.Add(new TextBlock
-        {
-            Text = "Excel workflow is intentionally not part of this primary FAT tab.",
-            Margin = new Thickness(0, 14, 0, 0),
-            FontSize = 10.6,
-            Foreground = TryFindResource("Muted") as Brush ?? Brushes.DimGray,
-            HorizontalAlignment = HorizontalAlignment.Center
-        });
+            panel.Children.Add(new ProgressBar
+            {
+                Height = 4,
+                IsIndeterminate = true,
+                BorderThickness = new Thickness(0),
+                Margin = new Thickness(0, 18, 0, 0),
+                Foreground = TryFindResource("Accent") as Brush ?? Brushes.RoyalBlue,
+                Background = new SolidColorBrush(Color.FromRgb(220, 231, 248))
+            });
+        }
 
-        card.Child = content;
+        card.Child = panel;
         root.Children.Add(card);
         return root;
     }
 
-    private void ProductionFatNavButton_Click(object sender, RoutedEventArgs e)
+    internal void ShowProductionFatBootstrapState(string message, bool isBusy)
     {
-        if (MainTabs.Items.Count <= NativeFatWorkspaceIndex)
+        if (!ProductionFatTabReady || _productionFatWindow is { IsLoaded: true })
             return;
 
-        MainTabs.SelectedIndex = NativeFatWorkspaceIndex;
-        QueueNativeFatNavigationGeometry();
-        SynchronizeProductionFatSelectedIed();
-        _productionFatWindow?.NotifyEmbeddedHostActivated();
+        NativeFatTab.Content = BuildProductionFatPermanentHost(message, isBusy);
     }
 
     private void ProductionFat_MainTabsSelectionChanged(object sender, SelectionChangedEventArgs e)
@@ -219,13 +191,13 @@ internal bool MountProductionFatWorkspace(IoListTestingWindow window, FrameworkE
     {
         ArgumentNullException.ThrowIfNull(window);
         ArgumentNullException.ThrowIfNull(surface);
-        if (!ProductionFatTabReady || _nativeFatTab == null)
+        if (!ProductionFatTabReady)
             return false;
 
         _productionFatWindow = window;
         _productionFatSurface = surface;
         surface.DataContext = window;
-        _nativeFatTab.Content = surface;
+        NativeFatTab.Content = surface;
         if (_persistentWorkbench != null)
             _persistentWorkbench.DockExpandedByWorkspace[NativeFatWorkspaceIndex] = true;
 
@@ -233,20 +205,13 @@ internal bool MountProductionFatWorkspace(IoListTestingWindow window, FrameworkE
         window.Closed += ProductionFatWindow_Closed;
         SynchronizeProductionFatSelectedIed();
 
-        MainTabs.SelectedIndex = NativeFatWorkspaceIndex;
+        // Passive mount: prewarming must never navigate, hide/show, activate, or steal
+        // focus from the operator's current Engineering destination.
+        window.RegisterEmbeddedHostCloseCleanup();
         QueueNativeFatNavigationGeometry();
 
-        // The legacy launcher hides Engineering before showing IoListTestingWindow.
-        // Once its proven central workspace is re-parented here, restore Engineering and
-        // keep the legacy Window loaded-but-hidden solely as the production controller owner.
-        IsEnabled = true;
-        if (!IsVisible)
-            Show();
-        if (WindowState == WindowState.Minimized)
-            WindowState = WindowState.Normal;
-        Activate();
-
-        SetStatus($"FAT ready in Engineering tab · {window.Project.Ieds.Count} IED · production auto-capture workflow.");
+        if (MainTabs.SelectedIndex == NativeFatWorkspaceIndex)
+            SetStatus($"FAT ready in Engineering tab · {window.Project.Ieds.Count} IED · production auto-capture workflow.");
         return true;
     }
 
@@ -258,8 +223,7 @@ internal void UnmountProductionFatWorkspace(IoListTestingWindow window)
         window.Closed -= ProductionFatWindow_Closed;
         _productionFatWindow = null;
         _productionFatSurface = null;
-        if (_nativeFatTab != null)
-            _nativeFatTab.Content = BuildProductionFatLauncher();
+        NativeFatTab.Content = BuildProductionFatPermanentHost();
     }
 
     private void ProductionFatWindow_Closed(object? sender, EventArgs e)
diff --git a/MainWindow.SharedSclWorkspace.cs b/MainWindow.SharedSclWorkspace.cs
index a7a0ef71b..1fcf55dd0 100644
--- a/MainWindow.SharedSclWorkspace.cs
+++ b/MainWindow.SharedSclWorkspace.cs
@@ -98,6 +98,11 @@ private void ApplyStaticDataSetSelection(Iec61850MonitorDevice device)
         // shared monitor to start and never changes acquisition method.
         LogStaticDataSetReportFeasibility(device);
         _ = ObserveInitialStaticReportEvidenceAsync(device);
+
+        // M2 permanent FAT host: authority establishment is also a readiness trigger.
+        // This covers SCL refresh on the same SelectedDevice, where PropertyChanged for
+        // SelectedDevice would otherwise not fire.
+        QueueProductionFatEngineeringBootstrap();
     }
 
     private void ClearSharedSignalSelection(Iec61850MonitorDevice device)
diff --git a/MainWindow.SharedStaticDataSetAuthority.cs b/MainWindow.SharedStaticDataSetAuthority.cs
new file mode 100644
index 000000000..bfeae00ab
--- /dev/null
+++ b/MainWindow.SharedStaticDataSetAuthority.cs
@@ -0,0 +1,27 @@
+using ArIED61850Tester.Models;
+using ArIED61850Tester.Services;
+
+namespace ArIED61850Tester;
+
+public partial class MainWindow
+{
+    /// <summary>
+    /// Reasserts the acquisition contract already chosen in Engineering when the FAT
+    /// projection is opened. FAT is a passive consumer of the shared static DataSet
+    /// workspace; this method deliberately performs no signal reselection, persistence,
+    /// reconnect, discovery, or RefreshComputed work.
+    /// </summary>
+    private void PreserveSharedStaticDataSetAuthority(Iec61850MonitorDevice device)
+    {
+        ArgumentNullException.ThrowIfNull(device);
+
+        Iec61850MonitoringModeRegistry.UseStaticDataSetReportOnly(device);
+        _sharedSclSelectionAuthorityDeviceIds.Add(device.DeviceId);
+        _sharedSclStaticDataSetAuthorityDeviceIds.Add(device.DeviceId);
+
+        AddLog(
+            "INFO",
+            "FAT",
+            $"Preserved Engineering Static DataSet report-only authority for {device.Name}; FAT did not reconnect, enable Hybrid, or start cyclic MMS polling.");
+    }
+}
diff --git a/MainWindow.xaml b/MainWindow.xaml
index 205519e97..af057d89c 100644
--- a/MainWindow.xaml
+++ b/MainWindow.xaml
@@ -121,8 +121,9 @@
                         <ColumnDefinition Width="*"/>
                         <ColumnDefinition Width="*"/>
                         <ColumnDefinition Width="*"/>
+                        <ColumnDefinition Width="*"/>
                     </Grid.ColumnDefinitions>
-                    <Border x:Name="WorkflowPill" Grid.ColumnSpan="6" Width="148" Height="34"
+                    <Border x:Name="WorkflowPill" Grid.ColumnSpan="7" Width="112" Height="34"
                             HorizontalAlignment="Left" VerticalAlignment="Center" CornerRadius="17"
                             Background="{StaticResource Accent}" RenderTransformOrigin="0.5,0.5">
                         <Border.RenderTransform>
@@ -151,6 +152,7 @@
                             </Border>
                         </StackPanel>
                     </Button>
+                    <Button x:Name="NavNativeFatButton" Grid.Column="6" Content="FAT" Tag="6" Click="NavButton_Click" Style="{StaticResource SegmentedNavButton}"/>
                 </Grid>
             </Border>
 
@@ -1684,7 +1686,12 @@
                     </DockPanel>
                 </Border>
             </TabItem>
-                </TabControl>
+
+            <!-- FAT: permanent seventh Engineering workspace host. Production presentation is mounted here. -->
+            <TabItem x:Name="NativeFatTab" Header="FAT">
+                <Grid/>
+            </TabItem>
+        </TabControl>
 
         <Border Grid.Row="2" Margin="0,10,0,0" Background="#EAF1FF" BorderBrush="#CFE0FF" BorderThickness="1" CornerRadius="14" Padding="11,7">
             <Grid>
diff --git a/MainWindow.xaml.cs b/MainWindow.xaml.cs
index 4f55a7528..00ae66f8a 100644
--- a/MainWindow.xaml.cs
+++ b/MainWindow.xaml.cs
@@ -620,7 +620,7 @@ private void NavButton_Click(object sender, RoutedEventArgs e)
     {
         if (sender is not Button button || !int.TryParse(button.Tag?.ToString(), out var index))
             return;
-        index = Math.Clamp(index, 0, 5);
+        index = Math.Clamp(index, 0, NativeFatWorkspaceIndex);
         MainTabs.SelectedIndex = index;
         UpdateNavigationVisuals(index, animate: true);
     }
@@ -654,7 +654,21 @@ private void UpdateNavigationVisuals(int index, bool animate)
         if (WorkflowPillTranslate == null)
             return;
 
-        var target = Math.Clamp(index, 0, 5) * 150d;
+        index = Math.Clamp(index, 0, NativeFatWorkspaceIndex);
+
+        // MainWindow is the single owner of all seven Engineering destinations.
+        // Keep the same density used by the proven workstation shell while allowing
+        // enough width for the Explorer label and the new canonical FAT sibling.
+        var availableWidth = ActualWidth > 0d ? ActualWidth : 1480d;
+        var shellWidth = availableWidth >= 1700d ? 1085d : availableWidth >= 1380d ? 995d : 805d;
+        WorkflowNavShell.Width = shellWidth;
+        WorkflowNavShell.MinWidth = shellWidth;
+
+        var contentWidth = Math.Max(0d, shellWidth - WorkflowNavShell.Padding.Left - WorkflowNavShell.Padding.Right);
+        var cellWidth = contentWidth / 7d;
+        WorkflowPill.Width = Math.Max(1d, cellWidth - 2d);
+        var target = index * cellWidth;
+
         if (animate)
         {
             var animation = new DoubleAnimation(target, TimeSpan.FromMilliseconds(190))
@@ -669,7 +683,16 @@ private void UpdateNavigationVisuals(int index, bool animate)
             WorkflowPillTranslate.X = target;
         }
 
-        var buttons = new[] { NavExplorerButton, NavLiveButton, NavEventsButton, NavAlarmButton, NavGooseButton, NavDiagnosticsButton };
+        var buttons = new[]
+        {
+            NavExplorerButton,
+            NavLiveButton,
+            NavEventsButton,
+            NavAlarmButton,
+            NavGooseButton,
+            NavDiagnosticsButton,
+            NavNativeFatButton
+        };
         for (var i = 0; i < buttons.Length; i++)
             buttons[i].Foreground = i == index ? Brushes.White : new SolidColorBrush(Color.FromRgb(71, 84, 103));
     }
diff --git a/SasOperationalUiPolicy.cs b/SasOperationalUiPolicy.cs
index cd9cd10d7..8c63dbce2 100644
--- a/SasOperationalUiPolicy.cs
+++ b/SasOperationalUiPolicy.cs
@@ -24,7 +24,7 @@ internal static class SasOperationalUiPolicy
     private static readonly ConditionalWeakTable<object, object> FilteredViews = new();
     private static readonly string[] NavigationButtonNames =
     {
-        "NavExplorerButton", "NavLiveButton", "NavEventsButton", "NavAlarmButton", "NavGooseButton", "NavDiagnosticsButton"
+        "NavExplorerButton", "NavLiveButton", "NavEventsButton", "NavAlarmButton", "NavGooseButton", "NavDiagnosticsButton", "NavNativeFatButton"
     };
 
     [ModuleInitializer]
diff --git a/Services/IoTesting/IoFatEngineeringWorkspaceProjectionService.cs b/Services/IoTesting/IoFatEngineeringWorkspaceProjectionService.cs
index d6c9bc89c..81eb06dd3 100644
--- a/Services/IoTesting/IoFatEngineeringWorkspaceProjectionService.cs
+++ b/Services/IoTesting/IoFatEngineeringWorkspaceProjectionService.cs
@@ -7,6 +7,7 @@ namespace ArIED61850Tester.Services.IoTesting;
 public sealed record IoFatEngineeringWorkspaceProjection(
     IoTestProject Project,
     IReadOnlyList<IoFatSourceInput> SourceInputs,
+    IReadOnlyList<IoFatDescribedSource> DescribedSources,
     IReadOnlyList<SclIedWorkspace> RuntimeWorkspaces);
 
 /// <summary>
@@ -36,12 +37,16 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
                 "No Engineering IED has an already-parsed SCL workspace with static DataSet members and source provenance.");
         }
 
-        var sourceInputs = usable
+        // Describe every candidate source once so conflicting Engineering authorities are
+        // detected before canonicalization. The resulting canonical source set is then used
+        // for the FAT projection/staging path so duplicate Explorer entries do not multiply
+        // static DataSet rows or repeat downstream workspace work.
+        var candidateSourceInputs = usable
             .Select(device => Path.GetFullPath(device.SclSourcePath))
             .Distinct(StringComparer.OrdinalIgnoreCase)
             .Select(path => new IoFatSourceInput(path, IoFatSourceKinds.Scl))
             .ToArray();
-        var described = await IoFatSourceWorkspaceService.DescribeAsync(sourceInputs, cancellationToken)
+        var described = await IoFatSourceWorkspaceService.DescribeAsync(candidateSourceInputs, cancellationToken)
             .ConfigureAwait(false);
         var descriptorByPath = described.ToDictionary(
             item => Path.GetFullPath(item.OriginalPath),
@@ -62,7 +67,21 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
             }
         }
 
-        var workspaceSources = usable
+        var canonicalDevices = CanonicalizeEngineeringDevices(usable, descriptorByPath);
+        var sourceInputs = canonicalDevices
+            .Select(device => Path.GetFullPath(device.SclSourcePath))
+            .Distinct(StringComparer.OrdinalIgnoreCase)
+            .Select(path => new IoFatSourceInput(path, IoFatSourceKinds.Scl))
+            .ToArray();
+        var canonicalDescribedSources = sourceInputs
+            .Select(input =>
+            {
+                var path = Path.GetFullPath(input.FilePath);
+                return new IoFatDescribedSource(descriptorByPath[path], path);
+            })
+            .ToArray();
+
+        var workspaceSources = canonicalDevices
             .Select(device =>
             {
                 var descriptor = descriptorByPath[Path.GetFullPath(device.SclSourcePath)];
@@ -74,14 +93,15 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
             .ToArray();
         var verification = FatSclWorkspaceImportService.Import(workspaceSources).Project;
 
-        var deviceByWorkspace = usable
-            .GroupBy(device => WorkspaceIdentity(device.SclWorkspace!), StringComparer.OrdinalIgnoreCase)
-            .ToDictionary(group => group.Key, group => group.First(), StringComparer.OrdinalIgnoreCase);
-        var descriptorByWorkspace = usable
-            .GroupBy(device => WorkspaceIdentity(device.SclWorkspace!), StringComparer.OrdinalIgnoreCase)
+        var deviceByWorkspace = canonicalDevices
             .ToDictionary(
-                group => group.Key,
-                group => descriptorByPath[Path.GetFullPath(group.First().SclSourcePath)],
+                device => WorkspaceIdentity(device.SclWorkspace!),
+                device => device,
+                StringComparer.OrdinalIgnoreCase);
+        var descriptorByWorkspace = canonicalDevices
+            .ToDictionary(
+                device => WorkspaceIdentity(device.SclWorkspace!),
+                device => descriptorByPath[Path.GetFullPath(device.SclSourcePath)],
                 StringComparer.OrdinalIgnoreCase);
 
         var plans = new List<IoTestIedPlan>();
@@ -120,8 +140,8 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
             plans.Add(plan);
         }
 
-        var sourceDescriptors = described
-            .Select(item => item.Source)
+        var sourceDescriptors = canonicalDevices
+            .Select(device => descriptorByPath[Path.GetFullPath(device.SclSourcePath)])
             .GroupBy(source => source.SourceId, StringComparer.OrdinalIgnoreCase)
             .Select(group => group.First())
             .OrderBy(source => source.SourceId, StringComparer.Ordinal)
@@ -144,7 +164,7 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
         project.SetSources(sourceDescriptors, sourceFingerprint);
         project.InitializeRuntimeNotifications();
 
-        var staticMemberCount = usable.Sum(device =>
+        var staticMemberCount = canonicalDevices.Sum(device =>
             device.SclWorkspace!.DesignModel.DataSets.Sum(dataSet => dataSet.Members.Count));
         if (project.SignalCount != verification.Signals.Count || project.SignalCount != staticMemberCount)
         {
@@ -155,7 +175,71 @@ public static async Task<IoFatEngineeringWorkspaceProjection> BuildAsync(
         return new IoFatEngineeringWorkspaceProjection(
             project,
             sourceInputs,
-            usable.Select(device => device.SclWorkspace!).ToArray());
+            canonicalDescribedSources,
+            canonicalDevices.Select(device => device.SclWorkspace!).ToArray());
+    }
+
+    private static IReadOnlyList<Iec61850MonitorDevice> CanonicalizeEngineeringDevices(
+        IReadOnlyCollection<Iec61850MonitorDevice> devices,
+        IReadOnlyDictionary<string, IoFatSourceDescriptor> descriptorByPath)
+    {
+        var canonical = new List<Iec61850MonitorDevice>();
+        foreach (var identityGroup in devices
+                     .GroupBy(device => WorkspaceIdentity(device.SclWorkspace!), StringComparer.OrdinalIgnoreCase)
+                     .OrderBy(group => group.Key, StringComparer.OrdinalIgnoreCase))
+        {
+            var candidates = identityGroup
+                .Select(device =>
+                {
+                    var path = Path.GetFullPath(device.SclSourcePath);
+                    return new
+                    {
+                        Device = device,
+                        Path = path,
+                        Descriptor = descriptorByPath[path]
+                    };
+                })
+                .ToArray();
+
+            var distinctHashes = candidates
+                .Select(candidate => candidate.Descriptor.Sha256)
+                .Distinct(StringComparer.OrdinalIgnoreCase)
+                .ToArray();
+            if (distinctHashes.Length > 1)
+            {
+                throw new InvalidDataException(
+                    $"Conflicting Engineering SCL sources define the same IED/AccessPoint '{identityGroup.Key}'. " +
+                    "FAT will not silently merge competing static DataSet authorities.");
+            }
+
+            var distinctEndpoints = candidates
+                .Select(candidate => candidate.Device.IpAddress?.Trim() ?? string.Empty)
+                .Where(endpoint => !string.IsNullOrWhiteSpace(endpoint))
+                .Distinct(StringComparer.OrdinalIgnoreCase)
+                .ToArray();
+            if (distinctEndpoints.Length > 1)
+            {
+                throw new InvalidDataException(
+                    $"Engineering exposes IED/AccessPoint '{identityGroup.Key}' through multiple endpoints ({string.Join(", ", distinctEndpoints)}). " +
+                    "FAT will not guess which physical IED owns the evidence session.");
+            }
+
+            // Exact same-content duplicates are already harmless according to the lower SCL
+            // importer contract. Collapse them here as well so projection counts, runtime
+            // workspaces and staged source files all share one canonical authority. Prefer the
+            // currently monitoring/connected Engineering device so FAT remains attached to the
+            // live session that the operator is already using.
+            var selected = candidates
+                .OrderByDescending(candidate => candidate.Device.IsMonitoring)
+                .ThenByDescending(candidate => candidate.Device.IsConnected)
+                .ThenBy(candidate => candidate.Descriptor.FileName, StringComparer.OrdinalIgnoreCase)
+                .ThenBy(candidate => candidate.Path, StringComparer.OrdinalIgnoreCase)
+                .ThenBy(candidate => candidate.Device.DeviceId, StringComparer.OrdinalIgnoreCase)
+                .First();
+            canonical.Add(selected.Device);
+        }
+
+        return canonical;
     }
 
     private static IoTestPointPlan ToPointPlan(
diff --git a/Services/IoTesting/IoFatProductionControllerAdapter.cs b/Services/IoTesting/IoFatProductionControllerAdapter.cs
new file mode 100644
index 000000000..d501009ed
--- /dev/null
+++ b/Services/IoTesting/IoFatProductionControllerAdapter.cs
@@ -0,0 +1,137 @@
+using ArIED61850Tester.Models.IoTesting;
+
+namespace ArIED61850Tester.Services.IoTesting;
+
+/// <summary>
+/// M4 transaction boundary between Engineering's mutable view context and the existing
+/// production FAT evidence engine. It freezes the operator-selected IED and exact capture
+/// scope before asynchronous connection preparation, then revalidates that lease before
+/// delegating to <see cref="IoTestMultiSessionCoordinator.Start(IoTestIedPlan?, IReadOnlyCollection{IoTestPointPlan}?)"/>.
+///
+/// This class deliberately does not parse SCL, discover/connect an IED, evaluate transitions,
+/// or write evidence. Those authorities remain with Engineering and IoTestSessionController.
+/// </summary>
+public static class IoFatProductionControllerAdapter
+{
+    public static IoFatCaptureTargetLease LatchStartTarget(
+        IoTestProject project,
+        IoTestIedPlan? ied)
+    {
+        ArgumentNullException.ThrowIfNull(project);
+        if (ied == null)
+            throw new InvalidOperationException("Select an imported IED first.");
+        if (!project.Ieds.Contains(ied))
+            throw new InvalidOperationException("The selected IED is no longer part of this FAT project.");
+
+        var preflight = IoTestSessionPreflight.Validate(ied);
+        if (!preflight.Succeeded)
+            throw new InvalidOperationException(preflight.Message);
+
+        var points = ied.TestPoints
+            .Where(point =>
+                point.WorkspaceSelected &&
+                point.IsIncludedInFat &&
+                point.TestEnabled &&
+                point.ImportReady)
+            .Distinct()
+            .Select(point => new IoFatCapturePointLease(
+                point,
+                IoTestPerIedProgressIdentity.PointConfigurationFingerprint(point)))
+            .ToArray();
+        if (points.Length == 0)
+        {
+            throw new InvalidOperationException(
+                "No import-ready operator-selected signal is available in the requested FAT capture scope.");
+        }
+
+        return new IoFatCaptureTargetLease(
+            ied,
+            IoTestPerIedProgressIdentity.IedKey(ied),
+            ied.LiveDeviceId?.Trim() ?? string.Empty,
+            points);
+    }
+
+    public static IoTestSessionActionResult StartLatched(
+        IoTestProject project,
+        IoTestMultiSessionCoordinator sessions,
+        IoFatCaptureTargetLease lease)
+    {
+        ArgumentNullException.ThrowIfNull(project);
+        ArgumentNullException.ThrowIfNull(sessions);
+        ArgumentNullException.ThrowIfNull(lease);
+
+        var validation = ValidateLease(project, lease);
+        if (!validation.Succeeded)
+            return validation;
+
+        // The existing production controller remains the only evidence writer. Passing the
+        // frozen scope prevents an async Engineering prepare from silently broadening or
+        // redirecting the session when Explorer selection/workspace membership changes.
+        return sessions.Start(lease.Ied, lease.CaptureScope);
+    }
+
+    public static IoTestSessionActionResult ValidateLease(
+        IoTestProject project,
+        IoFatCaptureTargetLease lease)
+    {
+        ArgumentNullException.ThrowIfNull(project);
+        ArgumentNullException.ThrowIfNull(lease);
+
+        if (!project.Ieds.Contains(lease.Ied))
+            return IoTestSessionActionResult.Failure("The latched FAT IED left the project while connection preparation was running. No evidence session was started.");
+
+        var currentIedIdentity = IoTestPerIedProgressIdentity.IedKey(lease.Ied);
+        if (!currentIedIdentity.Equals(lease.IedIdentity, StringComparison.OrdinalIgnoreCase))
+        {
+            return IoTestSessionActionResult.Failure(
+                "The latched IEC IED identity or endpoint changed during connection preparation. No evidence session was started.");
+        }
+
+        var currentDeviceId = lease.Ied.LiveDeviceId?.Trim() ?? string.Empty;
+        if (!string.IsNullOrWhiteSpace(lease.DeviceId) &&
+            !currentDeviceId.Equals(lease.DeviceId, StringComparison.OrdinalIgnoreCase))
+        {
+            return IoTestSessionActionResult.Failure(
+                "The Engineering DeviceId bound to the latched FAT IED changed during connection preparation. No evidence session was started.");
+        }
+
+        foreach (var pointLease in lease.Points)
+        {
+            var point = pointLease.Point;
+            if (!lease.Ied.TestPoints.Contains(point))
+            {
+                return IoTestSessionActionResult.Failure(
+                    "A latched FAT row left its owning IED during connection preparation. No evidence session was started.");
+            }
+            if (!point.WorkspaceSelected || !point.IsIncludedInFat || !point.TestEnabled || !point.ImportReady)
+            {
+                return IoTestSessionActionResult.Failure(
+                    $"The latched FAT row '{point.SignalName}' changed eligibility during connection preparation. Review the scope and press Start FAT again.");
+            }
+
+            var fingerprint = IoTestPerIedProgressIdentity.PointConfigurationFingerprint(point);
+            if (!fingerprint.Equals(pointLease.ConfigurationFingerprint, StringComparison.OrdinalIgnoreCase))
+            {
+                return IoTestSessionActionResult.Failure(
+                    $"The IEC/evidence configuration of latched FAT row '{point.SignalName}' changed during connection preparation. No evidence session was started.");
+            }
+        }
+
+        return IoTestSessionActionResult.Success(
+            $"Latched FAT target verified: {lease.Ied.IedName} · {lease.Points.Count} point(s).");
+    }
+}
+
+public sealed record IoFatCapturePointLease(
+    IoTestPointPlan Point,
+    string ConfigurationFingerprint);
+
+public sealed record IoFatCaptureTargetLease(
+    IoTestIedPlan Ied,
+    string IedIdentity,
+    string DeviceId,
+    IReadOnlyList<IoFatCapturePointLease> Points)
+{
+    public IReadOnlyList<IoTestPointPlan> CaptureScope { get; } =
+        Points.Select(point => point.Point).ToArray();
+}
diff --git a/Services/IoTesting/IoTestMultiSessionCoordinator.cs b/Services/IoTesting/IoTestMultiSessionCoordinator.cs
index 12037ac36..9c1d6236d 100644
--- a/Services/IoTesting/IoTestMultiSessionCoordinator.cs
+++ b/Services/IoTesting/IoTestMultiSessionCoordinator.cs
@@ -133,7 +133,9 @@ public IoTestSessionActionResult Start(
         if (ied == null)
             return IoTestSessionActionResult.Failure("Select an imported IED first.");
 
-        SelectContext(ied);
+        // M3: the caller latches the capture target before any asynchronous IED
+        // preparation. Starting that target must never steal the Explorer/view context
+        // if the operator selected another IED while preparation was in flight.
         IoTestSessionController controller;
         try
         {
@@ -154,16 +156,34 @@ public IoTestSessionActionResult Pause(string reason = "Paused by operator")
             controller => controller.Pause(reason),
             "No FAT evidence session is active for the selected IED.");
 
+    public IoTestSessionActionResult Pause(IoTestIedPlan? ied, string reason = "Paused by operator")
+        => TargetAction(
+            ied,
+            controller => controller.Pause(reason),
+            "No FAT evidence session is active for the latched IED.");
+
     public IoTestSessionActionResult Resume()
         => SelectedAction(
             controller => controller.Resume(),
             "No paused or interrupted FAT evidence session belongs to the selected IED.");
 
+    public IoTestSessionActionResult Resume(IoTestIedPlan? ied)
+        => TargetAction(
+            ied,
+            controller => controller.Resume(),
+            "No paused or interrupted FAT evidence session belongs to the latched IED.");
+
     public IoTestSessionActionResult Stop(string reason = "Stopped by operator")
         => SelectedAction(
             controller => controller.Stop(reason),
             "No FAT evidence session is active for the selected IED.");
 
+    public IoTestSessionActionResult Stop(IoTestIedPlan? ied, string reason = "Stopped by operator")
+        => TargetAction(
+            ied,
+            controller => controller.Stop(reason),
+            "No FAT evidence session is active for the latched IED.");
+
     public IoTestSessionActionResult StopAll(string reason = "Stopped by operator")
     {
         ThrowIfDisposed();
@@ -342,6 +362,26 @@ private IoTestSessionController GetOrCreateController(IoTestIedPlan ied)
         }
     }
 
+    private IoTestSessionActionResult TargetAction(
+        IoTestIedPlan? ied,
+        Func<IoTestSessionController, IoTestSessionActionResult> action,
+        string missingMessage)
+    {
+        ThrowIfDisposed();
+        if (ied == null)
+            return IoTestSessionActionResult.Failure(missingMessage);
+
+        IoTestSessionController? controller;
+        lock (_sync)
+            _controllers.TryGetValue(ied, out controller);
+        if (controller == null)
+            return IoTestSessionActionResult.Failure(missingMessage);
+
+        var result = action(controller);
+        RaiseProjectionProperties();
+        return result;
+    }
+
     private IoTestSessionActionResult SelectedAction(
         Func<IoTestSessionController, IoTestSessionActionResult> action,
         string missingMessage)
diff --git a/Services/IoTesting/IoTestPerIedProgressIdentity.cs b/Services/IoTesting/IoTestPerIedProgressIdentity.cs
index 62f220e5c..bb6511531 100644
--- a/Services/IoTesting/IoTestPerIedProgressIdentity.cs
+++ b/Services/IoTesting/IoTestPerIedProgressIdentity.cs
@@ -7,8 +7,9 @@ namespace ArIED61850Tester.Services.IoTesting;
 
 /// <summary>
 /// Stable identity contract for restoring persisted FAT progress onto a freshly imported plan.
-/// IED ownership is technical-key + IP scoped; point progress is accepted only when the
-/// evidence-critical configuration fingerprint still matches.
+/// IED ownership is IEC technical identity + endpoint scoped and, when both sides know it,
+/// additionally bound to the Engineering live DeviceId. Point progress is accepted only when
+/// the evidence-critical configuration fingerprint still matches.
 /// </summary>
 public static class IoTestPerIedProgressIdentity
 {
@@ -21,6 +22,44 @@ public static string IedKey(IoTestIedPlan ied)
     public static string IedKey(string? iedName, string? ipAddress)
         => $"{NormalizeIdentity(iedName)}|{NormalizeIp(ipAddress)}";
 
+    /// <summary>
+    /// Strong persisted-owner key for new snapshots. DeviceId is deliberately additive:
+    /// legacy snapshots did not persist it, so endpoint + IEC identity remains the migration
+    /// fallback only when either side does not yet know a DeviceId. Once both sides know it,
+    /// a mismatch is a hard fail-closed ownership boundary.
+    /// </summary>
+    public static string IedPersistenceAuthorityKey(IoTestIedPlan ied)
+    {
+        ArgumentNullException.ThrowIfNull(ied);
+        return IedPersistenceAuthorityKey(ied.LiveDeviceId, ied.IedName, ied.IpAddress);
+    }
+
+    public static string IedPersistenceAuthorityKey(
+        string? deviceId,
+        string? iedName,
+        string? ipAddress)
+        => $"{NormalizeIdentity(deviceId)}|{NormalizeIdentity(iedName)}|{NormalizeIp(ipAddress)}";
+
+    public static bool PersistedIedOwnershipMatches(IoTestIedPlan currentIed, JsonElement savedIed)
+    {
+        ArgumentNullException.ThrowIfNull(currentIed);
+
+        var savedName = OptionalString(savedIed, "iedName", string.Empty);
+        var savedIp = OptionalString(savedIed, "ipAddress", string.Empty);
+        if (!IedKey(currentIed).Equals(IedKey(savedName, savedIp), StringComparison.Ordinal))
+            return false;
+
+        var currentDeviceId = NormalizeIdentity(currentIed.LiveDeviceId);
+        var savedDeviceId = NormalizeIdentity(OptionalString(savedIed, "liveDeviceId", string.Empty));
+
+        // Backward compatibility is intentionally one-way: old snapshots without DeviceId
+        // may still restore through exact IEC identity + IP + point fingerprints. As soon as
+        // both snapshots/current Engineering context carry DeviceId, it becomes mandatory.
+        return currentDeviceId.Length == 0 ||
+               savedDeviceId.Length == 0 ||
+               currentDeviceId.Equals(savedDeviceId, StringComparison.Ordinal);
+    }
+
     public static string PointConfigurationFingerprint(IoTestPointPlan point)
         => PointConfigurationFingerprintCore(point, includeSignalAddress: true);
 
diff --git a/Services/IoTesting/IoTestProjectPersistenceService.cs b/Services/IoTesting/IoTestProjectPersistenceService.cs
index 9bfd1e55f..008e34a90 100644
--- a/Services/IoTesting/IoTestProjectPersistenceService.cs
+++ b/Services/IoTesting/IoTestProjectPersistenceService.cs
@@ -99,6 +99,38 @@ public static async Task<IoTestWorkspaceOpenResult> OpenSourcesAsync(
         cancellationToken.ThrowIfCancellationRequested();
 
         var described = await IoFatSourceWorkspaceService.DescribeAsync(sourceInputs, cancellationToken).ConfigureAwait(false);
+        return await OpenDescribedSourcesAsync(
+            importedProject,
+            session,
+            described,
+            localProjectsRoot,
+            evidenceRoot,
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// Opens a FAT workspace from a source set whose SHA-256 provenance has already been
+    /// calculated by an upstream authority in the same launch flow. This avoids hashing the
+    /// same SCL bytes again between Engineering projection and persistence. Staging still
+    /// reads every source and verifies its bytes against the supplied SHA-256 before the
+    /// canonical local copy is accepted, so this optimization never weakens content identity.
+    /// </summary>
+    public static async Task<IoTestWorkspaceOpenResult> OpenDescribedSourcesAsync(
+        IoTestProject importedProject,
+        IoTestSessionController session,
+        IReadOnlyCollection<IoFatDescribedSource> describedSources,
+        string localProjectsRoot,
+        string evidenceRoot,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(importedProject);
+        ArgumentNullException.ThrowIfNull(session);
+        ArgumentNullException.ThrowIfNull(describedSources);
+        if (describedSources.Count == 0)
+            throw new InvalidDataException("A FAT workspace must contain at least one source file.");
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var described = describedSources.ToArray();
         IoFatSourceIdentity.AttachOrValidate(importedProject, described.Select(source => source.Source).ToArray());
 
         var localDirectory = ProjectDirectory(localProjectsRoot, importedProject);
@@ -572,6 +604,7 @@ private void SaveFromTimer()
                     ReviewerComment = point.ReviewerComment
                 }).ToList())
             {
+                LiveDeviceId = ied.LiveDeviceId,
                 LatestComtradeFiles = ied.LatestComtradeFiles,
                 LatestComtradeRemotePath = ied.LatestComtradeRemotePath,
                 LatestComtradeCompleteness = ied.LatestComtradeCompleteness,
@@ -987,6 +1020,7 @@ private sealed record IoTestIedData(
         string Switchgear,
         List<IoTestPointData> TestPoints)
     {
+        public string LiveDeviceId { get; init; } = string.Empty;
         public string LatestComtradeFiles { get; init; } = string.Empty;
         public string LatestComtradeRemotePath { get; init; } = string.Empty;
         public string LatestComtradeCompleteness { get; init; } = string.Empty;
diff --git a/Services/IoTesting/IoTestWorkspaceBootstrapService.cs b/Services/IoTesting/IoTestWorkspaceBootstrapService.cs
index 72d02b943..d444880ab 100644
--- a/Services/IoTesting/IoTestWorkspaceBootstrapService.cs
+++ b/Services/IoTesting/IoTestWorkspaceBootstrapService.cs
@@ -45,6 +45,36 @@ public static async Task<IoTestWorkspaceLaunchResult> OpenSourcesAsync(
         ArgumentNullException.ThrowIfNull(sessionFactory);
 
         var described = await IoFatSourceWorkspaceService.DescribeAsync(sourceInputs, cancellationToken).ConfigureAwait(false);
+        return await OpenDescribedSourcesAsync(
+            importedProject,
+            described,
+            localProjectsRoot,
+            evidenceRoot,
+            sessionFactory,
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// Continues FAT bootstrap with source identities already SHA-256-described upstream.
+    /// The persistence layer accepts the same immutable descriptions and still verifies the
+    /// bytes again while staging them, so this removes redundant full-file hashing without
+    /// weakening the source/evidence boundary.
+    /// </summary>
+    public static async Task<IoTestWorkspaceLaunchResult> OpenDescribedSourcesAsync(
+        IoTestProject importedProject,
+        IReadOnlyCollection<IoFatDescribedSource> describedSources,
+        string localProjectsRoot,
+        string evidenceRoot,
+        Func<IoTestProject, string, IoTestSessionController> sessionFactory,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(importedProject);
+        ArgumentNullException.ThrowIfNull(describedSources);
+        ArgumentNullException.ThrowIfNull(sessionFactory);
+        if (describedSources.Count == 0)
+            throw new InvalidDataException("A FAT workspace must contain at least one source file.");
+
+        var described = describedSources.ToArray();
         IoFatSourceIdentity.AttachOrValidate(importedProject, described.Select(source => source.Source).ToArray());
 
         var localDirectory = ProjectDirectory(localProjectsRoot, importedProject);
@@ -61,21 +91,23 @@ public static async Task<IoTestWorkspaceLaunchResult> OpenSourcesAsync(
         {
             if (!string.IsNullOrWhiteSpace(restoreSnapshotPath) && File.Exists(restoreSnapshotPath))
             {
+                // Isolate the canonical persisted snapshot BEFORE attempting selective restore.
+                // Otherwise a rejected/invalid partial restore could fall through to
+                // IoTestWorkspacePersistence.OpenDescribedSourcesAsync(), which is allowed to restore a
+                // matching snapshot wholesale. Engineering is the fresh plan authority here:
+                // the old snapshot is read-only evidence input, never a replacement project.
+                if (restoreSnapshotPath.Equals(snapshotPath, StringComparison.OrdinalIgnoreCase))
+                {
+                    Directory.CreateDirectory(localDirectory);
+                    File.Move(snapshotPath, backupPath, true);
+                    movedSnapshot = true;
+                    restoreSnapshotPath = backupPath;
+                }
+
                 try
                 {
                     ApplySnapshotProgress(importedProject, restoreSnapshotPath);
                     restored = true;
-
-                    // The snapshot at the canonical current path is temporarily moved so
-                    // persistence cannot replace the freshly imported IEC model wholesale.
-                    // A compatible SCL snapshot discovered under an older staging/project
-                    // identity is read-only input; the new current path is saved normally.
-                    if (restoreSnapshotPath.Equals(snapshotPath, StringComparison.OrdinalIgnoreCase))
-                    {
-                        Directory.CreateDirectory(localDirectory);
-                        File.Move(snapshotPath, backupPath, true);
-                        movedSnapshot = true;
-                    }
                 }
                 catch (Exception ex) when (ex is IOException or JsonException or InvalidDataException)
                 {
@@ -86,10 +118,10 @@ public static async Task<IoTestWorkspaceLaunchResult> OpenSourcesAsync(
             var session = sessionFactory(importedProject, evidenceRoot);
             try
             {
-                var opened = await IoTestWorkspacePersistence.OpenSourcesAsync(
+                var opened = await IoTestWorkspacePersistence.OpenDescribedSourcesAsync(
                     importedProject,
                     session,
-                    sourceInputs,
+                    described,
                     localProjectsRoot,
                     evidenceRoot,
                     cancellationToken).ConfigureAwait(false);
@@ -248,7 +280,8 @@ private static void ApplySnapshotProgress(IoTestProject project, string snapshot
             var iedKey = IoTestPerIedProgressIdentity.IedKey(ied);
             if (!currentIedIdentityCounts.TryGetValue(iedKey, out var currentIdentityCount) ||
                 currentIdentityCount != 1 ||
-                !savedIedsByIdentity.TryGetValue(iedKey, out var savedIed))
+                !savedIedsByIdentity.TryGetValue(iedKey, out var savedIed) ||
+                !IoTestPerIedProgressIdentity.PersistedIedOwnershipMatches(ied, savedIed))
             {
                 continue;
             }
@@ -443,8 +476,11 @@ private static void RestoreMissingManualWorkspaceRows(
             var savedIed = savedIeds.FirstOrDefault(candidate =>
                 OptionalString(candidate, "iedName", string.Empty).Equals(ied.IedName, StringComparison.OrdinalIgnoreCase) &&
                 OptionalString(candidate, "ipAddress", string.Empty).Equals(ied.IpAddress, StringComparison.OrdinalIgnoreCase));
-            if (savedIed.ValueKind != JsonValueKind.Object)
+            if (savedIed.ValueKind != JsonValueKind.Object ||
+                !IoTestPerIedProgressIdentity.PersistedIedOwnershipMatches(ied, savedIed))
+            {
                 continue;
+            }
 
             var existingIds = ied.TestPoints
                 .Select(point => point.TestPointId)
@@ -541,8 +577,11 @@ private static void RestoreIedLevelEvidence(IoTestProject project, IReadOnlyList
             var saved = savedIeds.FirstOrDefault(candidate =>
                 OptionalString(candidate, "iedName", string.Empty).Equals(ied.IedName, StringComparison.OrdinalIgnoreCase) &&
                 OptionalString(candidate, "ipAddress", string.Empty).Equals(ied.IpAddress, StringComparison.OrdinalIgnoreCase));
-            if (saved.ValueKind != JsonValueKind.Object)
+            if (saved.ValueKind != JsonValueKind.Object ||
+                !IoTestPerIedProgressIdentity.PersistedIedOwnershipMatches(ied, saved))
+            {
                 continue;
+            }
 
             ied.LatestComtradeFiles = OptionalString(saved, "latestComtradeFiles", string.Empty);
             ied.LatestComtradeRemotePath = OptionalString(saved, "latestComtradeRemotePath", string.Empty);
diff --git a/docs/FAT_ENGINEERING_WORKSTATION_CONTRACT.md b/docs/FAT_ENGINEERING_WORKSTATION_CONTRACT.md
new file mode 100644
index 000000000..0963aba70
--- /dev/null
+++ b/docs/FAT_ENGINEERING_WORKSTATION_CONTRACT.md
@@ -0,0 +1,112 @@
+# ARSAS FAT Engineering Workstation Contract
+
+This document is the architectural freeze for the Engineering-integrated FAT workflow. Changes that violate these invariants must not be merged, even if a visual patch appears to fix one field symptom.
+
+## Ownership
+
+- `MainWindow` owns the persistent Engineering workstation shell, navigation, global IED Explorer, and shared Command Dock.
+- Engineering owns IEC 61850 device identity, `SelectedDevice`, parsed `SclIedWorkspace`, static DataSet selection, live signal definitions, live points, MMS/report acquisition, and connection lifecycle.
+- The production FAT controller owns FAT preflight, session lifecycle, capture semantics, V1/V2 evidence, completion/result semantics, and FAT persistence.
+- FAT presentation must not own a second IEC 61850 parser, discovery session, MMS connection, live polling loop, or command runtime.
+- Report presentation must reuse the production FAT report definition; preview and PDF/print must not diverge into separate report models.
+
+## Primary user flow
+
+`Open SCL -> Connect/monitor IED -> select IED in persistent Explorer -> FAT`
+
+The first visible FAT frame must be the production FAT workspace for the selected Engineering IED. The primary flow must not require `Open SCL for FAT`, a launcher card, a second SCL parse, or a separate operator-visible FAT window.
+
+## Navigation invariant
+
+Engineering navigation has exactly seven destinations in this order:
+
+1. IEC 61850 Explorer
+2. Live Monitor
+3. Event Log
+4. Alarm
+5. GOOSE
+6. Diagnostics
+7. FAT
+
+There must be one canonical navigation owner. Do not reintroduce a second `ModuleInitializer` / `ApplicationIdle` parity layer that fights the canonical navigation state.
+
+M1 locks FAT as a literal seventh XAML sibling using the same `SegmentedNavButton` contract. Every responsive/navigation presentation layer must consume the same seven destinations: MainWindow index range is `0..6`, selection geometry divides the available navigation width into seven equal slots, and ballistic navigation styling includes `NavNativeFatButton`. Regression tests must fail if any of these layers silently falls back to six-slot behavior.
+
+## Multi-IED context invariant
+
+When FAT is idle, the viewed FAT IED follows Engineering `SelectedDevice`.
+
+When a FAT session starts or continues, the capture target is latched to that session IED. Changing the Explorer selection may change the viewed IED but must never redirect an active capture/evidence transaction to another IED.
+
+Persisted FAT state is reconciled by stable device/IEC identity. Background reconciliation is read/reconcile-only and must not manufacture new evidence.
+
+M3 is CODE VERIFIED and CI VERIFIED around this split: selected context remains an operator view/control projection, while evidence actions resolve the owning active IED session. Regression coverage exercises concurrent IED sessions while the selected context changes, and requires independent evidence journals/device routing. FIELD VERIFIED remains intentionally open until physical-IED acceptance.
+
+## Persistence authority invariant
+
+A persisted FAT result belongs to an IED only after its IEC technical identity and endpoint match. New M5 snapshots also persist the Engineering `LiveDeviceId`; when both the current Engineering IED and the saved snapshot know a DeviceId, a DeviceId mismatch is a hard fail-closed boundary.
+
+Legacy snapshots without DeviceId may migrate only through the exact IEC identity + endpoint path and the existing evidence-critical point configuration fingerprint. A persistence candidate may contribute prior progress/evidence to the freshly projected Engineering plan, but it must never replace that plan wholesale.
+
+Persistence reconciliation is not an evidence-producing operation. It may adopt already-persisted V1/V2/transition/result state into the matching current IED, but it must not append or manufacture evidence journal records. Production evidence creation remains owned by explicit FAT session/capture operations.
+
+The Engineering bootstrap must reconcile the isolated snapshot before creating its production FAT session controller. This ordering is intentional: reconciliation has no active evidence writer to call, then the normal production session/persistence controller is attached after the current Engineering plan has been safely reconciled.
+
+## Static DataSet authority invariant
+
+Automatic Engineering FAT uses the authoritative static DataSet projection from the already parsed `SclIedWorkspace`.
+
+Synthesized `scl-manual-*` aliases must not be allowed to create duplicate enabled ownership of the same primary live leaf in automatic static mode. The strict duplicate-live-reference preflight guard remains enabled; invalid duplicate scope is cleaned up rather than weakening the guard.
+
+Manual-only projects keep their manual-row behavior.
+
+## FAT presentation invariant
+
+The final Engineering FAT center must reuse the exact production FAT grid presentation rather than an approximation or a second native FAT grid implementation.
+
+Target reusable structure:
+
+- `FatWorkspaceView`: canonical production grid/workflow presentation.
+- `FatReportPreviewView`: canonical production A4 preview presentation.
+- `IoListTestingWindow`: legacy standalone shell that may host the reusable views for compatibility, but must not be the presentation/lifecycle owner of the Engineering FAT tab.
+
+The Engineering FAT tab must own a permanent FAT visual surface. Selecting FAT should only switch workstation content; it must not require `Window.Show -> Loaded -> detach/reparent`.
+
+## Report preview invariant
+
+Default center = production FAT grid.
+
+`Print/Report Preview` switches the same FAT center to the canonical paged A4 preview. The persistent IED Explorer and shared Command Dock remain visible. Returning from preview restores the same grid instance/state, including selection, scroll position, V1/V2 state, and active session context.
+
+## Protected production behavior
+
+Refactoring presentation must not change:
+
+- Select/SBO/Operate sequencing or control safety behavior.
+- `ctlModel`, interlock/synchrocheck/Test, CommandTermination semantics.
+- RCB runtime behavior.
+- GOOSE capture.
+- MMS/report/live-monitor acquisition semantics.
+- Production FAT preflight, auto-capture, evidence, completion/result, and persistence semantics.
+
+## Delivery gates
+
+A milestone is not called fixed until it has three statuses:
+
+- CODE VERIFIED
+- CI VERIFIED
+- FIELD VERIFIED
+
+Once a field gate is accepted, later milestones should not change that layer unless a regression test demonstrates a real dependency.
+
+## Current migration order
+
+1. M0 Architecture freeze and regression locks.
+2. M1 Canonical seven-destination shell/navigation.
+3. M2 Permanent FAT surface; remove primary launcher/black transition.
+4. M3 Selected-IED view synchronization.
+5. M4 Production controller adapter and latched capture context.
+6. M5 Evidence/persistence reconciliation.
+7. M6 Canonical in-place A4 report preview.
+8. M7 Remove donor-window/parity/experimental duplicate presentation code.
+9. M8 Physical-IED acceptance.
diff --git a/docs/FAT_M2_PERMANENT_HOST_VERIFICATION.md b/docs/FAT_M2_PERMANENT_HOST_VERIFICATION.md
new file mode 100644
index 000000000..9cd780163
--- /dev/null
+++ b/docs/FAT_M2_PERMANENT_HOST_VERIFICATION.md
@@ -0,0 +1,18 @@
+# FAT M2 — Permanent Production Host Verification
+
+M2 removes the FAT launcher as the primary Engineering experience and keeps the proven production FAT surface as the single presentation/engine authority.
+
+## Code contract
+
+- `MainWindow` owns the canonical FAT tab and permanent host slot.
+- Engineering static DataSet authority may prewarm FAT before the operator navigates to FAT.
+- Prewarm does not change `MainTabs.SelectedIndex`, call `MainWindow.Show()`, call `Activate()`, or steal focus.
+- `IoListTestingWindow` remains a temporary compatibility donor/controller owner only; it is prepared transparent, off-screen, non-activating, and absent from the taskbar before `Window.Show()`.
+- `MainWindow` remains visible while the embedded donor is initialized.
+- The embedded center is the exact production workspace detached from `IoListTestingWindow`; `InstallFatV2WorkspaceUx()` and the existing report-preview installer remain the presentation sources.
+- No `Open SCL for FAT` or `Open ARSAS Project` launcher is exposed in the canonical FAT tab.
+- Engineering continues to supply the already parsed SCL/static DataSet and shared live runtime; M2 does not introduce a second parser or acquisition authority.
+
+## Verification gate
+
+M2 is **code complete** only when the permanent-host regression tests compile and pass. It is **CI verified** only when the staging head passes the repository Build ARSAS, IO List Testing, and SV evidence validation workflows. Physical no-flicker behavior remains a field-verification item and must not be claimed from CI alone.
diff --git a/tests/ARSAS.Tests/AlarmAnnunciatorIntegrationRegressionTests.cs b/tests/ARSAS.Tests/AlarmAnnunciatorIntegrationRegressionTests.cs
index f7811ae2f..78926fd52 100644
--- a/tests/ARSAS.Tests/AlarmAnnunciatorIntegrationRegressionTests.cs
+++ b/tests/ARSAS.Tests/AlarmAnnunciatorIntegrationRegressionTests.cs
@@ -5,7 +5,7 @@ namespace ARSAS.Tests;
 public sealed class AlarmAnnunciatorIntegrationRegressionTests
 {
     [Fact]
-    public void Header_UsesSixWorkflowDestinations_WithoutDuplicateRuntimeStatusChips()
+    public void Header_UsesSevenWorkflowDestinations_WithoutDuplicateRuntimeStatusChips()
     {
         var document = XDocument.Parse(File.ReadAllText(FindRepoFile("MainWindow.xaml")));
         XNamespace p = "http://schemas.microsoft.com/winfx/2006/xaml/presentation";
@@ -20,7 +20,7 @@ public void Header_UsesSixWorkflowDestinations_WithoutDuplicateRuntimeStatusChip
         var navColumns = navGrid.Element(p + "Grid.ColumnDefinitions")?
             .Elements(p + "ColumnDefinition").Count() ?? 0;
 
-        Assert.Equal(6, navColumns);
+        Assert.Equal(7, navColumns);
         Assert.Contains(navGrid.Descendants(p + "Button"), button =>
             (string?)button.Attribute(x + "Name") == "NavAlarmButton" &&
             (string?)button.Attribute("Tag") == "3" &&
@@ -31,6 +31,10 @@ public void Header_UsesSixWorkflowDestinations_WithoutDuplicateRuntimeStatusChip
         Assert.Contains(navGrid.Descendants(p + "Button"), button =>
             (string?)button.Attribute(x + "Name") == "NavDiagnosticsButton" &&
             (string?)button.Attribute("Tag") == "5");
+        Assert.Contains(navGrid.Descendants(p + "Button"), button =>
+            (string?)button.Attribute(x + "Name") == "NavNativeFatButton" &&
+            (string?)button.Attribute("Tag") == "6" &&
+            (string?)button.Attribute("Content") == "FAT");
 
         var headerSource = header.ToString(SaveOptions.DisableFormatting);
         Assert.DoesNotContain("ConnectionInsightText", headerSource, StringComparison.Ordinal);
@@ -46,7 +50,7 @@ public void BallisticNavbar_AppliesTheSameCapsuleContractToAlarm()
     {
         var source = File.ReadAllText(FindRepoFile("SasOperationalUiPolicy.cs"));
 
-        Assert.Contains("\"NavExplorerButton\", \"NavLiveButton\", \"NavEventsButton\", \"NavAlarmButton\", \"NavGooseButton\", \"NavDiagnosticsButton\"", source, StringComparison.Ordinal);
+        Assert.Contains("\"NavExplorerButton\", \"NavLiveButton\", \"NavEventsButton\", \"NavAlarmButton\", \"NavGooseButton\", \"NavDiagnosticsButton\", \"NavNativeFatButton\"", source, StringComparison.Ordinal);
         Assert.Contains("UpdateNavigation(buttons, tabs.SelectedIndex", source, StringComparison.Ordinal);
         Assert.Contains("buttons[index].Background = selected ? AccentGradient()", source, StringComparison.Ordinal);
     }
@@ -122,17 +126,20 @@ public void Annunciator_FlashAndAckWorkAreBoundedToVisibleIedOrIedRail()
     }
 
     [Fact]
-    public void NavigationAndDiagnosticsUseSixTabGeometry()
+    public void NavigationAndDiagnosticsUseSevenTabGeometry()
     {
         var navigation = File.ReadAllText(FindRepoFile("MainWindow.NavigationLayoutFix.cs"));
         var main = File.ReadAllText(FindRepoFile("MainWindow.xaml.cs"));
+        var nativeFat = File.ReadAllText(FindRepoFile("MainWindow.NativeFatWorkspace.cs"));
 
-        Assert.Contains("contentWidth / 6d", navigation, StringComparison.Ordinal);
-        Assert.Contains("Math.Clamp(tabs.SelectedIndex, 0, 5)", navigation, StringComparison.Ordinal);
+        Assert.Contains("contentWidth / 7d", navigation, StringComparison.Ordinal);
+        Assert.Contains("Math.Clamp(tabs.SelectedIndex, 0, 6)", navigation, StringComparison.Ordinal);
         Assert.Contains("NavAlarmButton", navigation, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton", navigation, StringComparison.Ordinal);
         Assert.Contains("MainTabs.SelectedIndex == 4", main, StringComparison.Ordinal);
         Assert.Contains("MainTabs.SelectedIndex == 5", main, StringComparison.Ordinal);
-        Assert.Contains("Math.Clamp(index, 0, 5)", main, StringComparison.Ordinal);
+        Assert.Contains("private const int NativeFatWorkspaceIndex = 6", nativeFat, StringComparison.Ordinal);
+        Assert.Contains("Math.Clamp(index, 0, NativeFatWorkspaceIndex)", main, StringComparison.Ordinal);
     }
 
     [Fact]
diff --git a/tests/ARSAS.Tests/IoFatM4ProductionControllerAdapterRegressionTests.cs b/tests/ARSAS.Tests/IoFatM4ProductionControllerAdapterRegressionTests.cs
new file mode 100644
index 000000000..87888f7f6
--- /dev/null
+++ b/tests/ARSAS.Tests/IoFatM4ProductionControllerAdapterRegressionTests.cs
@@ -0,0 +1,236 @@
+using ArIED61850Tester.Models;
+using ArIED61850Tester.Models.IoTesting;
+using ArIED61850Tester.Services.IoTesting;
+
+namespace ARSAS.Tests;
+
+public sealed class IoFatM4ProductionControllerAdapterRegressionTests
+{
+    [Fact]
+    public void LatchedStart_DoesNotFollowExplorerSelection_AndKeepsFrozenScope()
+    {
+        var fixture = CreateFixture();
+        fixture.PointA2.WorkspaceSelected = false;
+        new IoTestLiveBindingService().Bind(fixture.Project, fixture.Devices);
+
+        using var primary = NewController(fixture.Project, fixture.Devices, fixture.Root);
+        using var sessions = new IoTestMultiSessionCoordinator(fixture.Project, primary);
+        sessions.SelectContext(fixture.IedA);
+
+        var lease = IoFatProductionControllerAdapter.LatchStartTarget(fixture.Project, fixture.IedA);
+        Assert.Single(lease.Points);
+        Assert.Same(fixture.PointA1, lease.Points[0].Point);
+
+        // Simulate changes that can occur while Engineering connection preparation awaits:
+        // the global Explorer now views B and a second row becomes selected. Neither may
+        // redirect or widen the already-latched production evidence transaction.
+        sessions.SelectContext(fixture.IedB);
+        fixture.PointA2.WorkspaceSelected = true;
+
+        var start = IoFatProductionControllerAdapter.StartLatched(fixture.Project, sessions, lease);
+
+        Assert.True(start.Succeeded, start.Message);
+        Assert.Same(fixture.IedB, sessions.SelectedIed);
+        Assert.True(sessions.IsIedSessionActive(fixture.IedA));
+        Assert.False(sessions.IsIedSessionActive(fixture.IedB));
+        Assert.Contains(" / 1 complete", primary.ProgressText, StringComparison.Ordinal);
+
+        Assert.True(sessions.Stop(fixture.IedA, "test cleanup").Succeeded);
+    }
+
+    [Fact]
+    public void LatchedStart_FailsClosed_WhenEngineeringDeviceIdDriftsBeforeStart()
+    {
+        var fixture = CreateFixture();
+        new IoTestLiveBindingService().Bind(fixture.Project, fixture.Devices);
+
+        using var primary = NewController(fixture.Project, fixture.Devices, fixture.Root);
+        using var sessions = new IoTestMultiSessionCoordinator(fixture.Project, primary);
+        var lease = IoFatProductionControllerAdapter.LatchStartTarget(fixture.Project, fixture.IedA);
+        Assert.Equal("device-a", lease.DeviceId, ignoreCase: true);
+
+        fixture.IedA.ApplyLiveDeviceBinding(
+            "device-b",
+            "simulated Engineering binding drift",
+            isConnected: true,
+            isMonitoring: true);
+
+        var start = IoFatProductionControllerAdapter.StartLatched(fixture.Project, sessions, lease);
+
+        Assert.False(start.Succeeded);
+        Assert.Contains("DeviceId", start.Message, StringComparison.OrdinalIgnoreCase);
+        Assert.False(sessions.HasActiveSessions);
+        Assert.Empty(Directory.GetFiles(fixture.Root, "*.evidence.jsonl", SearchOption.AllDirectories));
+    }
+
+    [Fact]
+    public void LatchedStart_FailsClosed_WhenFrozenRowEligibilityChanges()
+    {
+        var fixture = CreateFixture();
+        new IoTestLiveBindingService().Bind(fixture.Project, fixture.Devices);
+
+        using var primary = NewController(fixture.Project, fixture.Devices, fixture.Root);
+        using var sessions = new IoTestMultiSessionCoordinator(fixture.Project, primary);
+        var lease = IoFatProductionControllerAdapter.LatchStartTarget(fixture.Project, fixture.IedA);
+
+        fixture.PointA1.WorkspaceSelected = false;
+        var start = IoFatProductionControllerAdapter.StartLatched(fixture.Project, sessions, lease);
+
+        Assert.False(start.Succeeded);
+        Assert.Contains("eligibility", start.Message, StringComparison.OrdinalIgnoreCase);
+        Assert.False(sessions.HasActiveSessions);
+        Assert.Empty(Directory.GetFiles(fixture.Root, "*.evidence.jsonl", SearchOption.AllDirectories));
+    }
+
+    [Fact]
+    public void ContinuePauseAndStop_UseExplicitIedOwner_NotCurrentExplorerProjection()
+    {
+        var fixture = CreateFixture();
+        new IoTestLiveBindingService().Bind(fixture.Project, fixture.Devices);
+
+        using var primary = NewController(fixture.Project, fixture.Devices, fixture.Root);
+        using var sessions = new IoTestMultiSessionCoordinator(fixture.Project, primary);
+        sessions.SelectContext(fixture.IedA);
+        var lease = IoFatProductionControllerAdapter.LatchStartTarget(fixture.Project, fixture.IedA);
+        Assert.True(IoFatProductionControllerAdapter.StartLatched(fixture.Project, sessions, lease).Succeeded);
+
+        Assert.True(sessions.Pause(fixture.IedA, "pause A").Succeeded);
+        sessions.SelectContext(fixture.IedB);
+
+        // Continue A while B is the viewed IED. The selected projection must remain B,
+        // while the active production controller continues its already-owned A session.
+        var resume = sessions.Resume(fixture.IedA);
+        Assert.True(resume.Succeeded, resume.Message);
+        Assert.Same(fixture.IedB, sessions.SelectedIed);
+        Assert.True(sessions.IsIedSessionActive(fixture.IedA));
+        Assert.False(sessions.IsIedSessionActive(fixture.IedB));
+
+        var stop = sessions.Stop(fixture.IedA, "stop A");
+        Assert.True(stop.Succeeded, stop.Message);
+        Assert.False(sessions.HasActiveSessions);
+        Assert.Same(fixture.IedB, sessions.SelectedIed);
+    }
+
+    private static Fixture CreateFixture()
+    {
+        var pointA1 = Point("A-1", "IED_A", "192.168.81.10", "IED_ALD0/GGIO1.Ind1.stVal");
+        var pointA2 = Point("A-2", "IED_A", "192.168.81.10", "IED_ALD0/GGIO1.Ind2.stVal");
+        var pointB = Point("B-1", "IED_B", "192.168.81.11", "IED_BLD0/GGIO1.Ind1.stVal");
+        var iedA = Ied("IED_A", "192.168.81.10", pointA1, pointA2);
+        var iedB = Ied("IED_B", "192.168.81.11", pointB);
+        var project = new IoTestProject
+        {
+            ProjectId = "M4-CAPTURE-LEASE",
+            SchemaVersion = "ARSAS-FAT-IO-1.0",
+            ProjectName = "M4 Production Adapter",
+            SourceWorkbookName = "M4.xlsx",
+            SourceWorkbookSha256 = new string('b', 64),
+            Ieds = { iedA, iedB }
+        };
+        project.InitializeRuntimeNotifications();
+
+        var deviceA = Device("device-a", iedA, pointA1, pointA2);
+        var deviceB = Device("device-b", iedB, pointB);
+        var devices = new[] { deviceA, deviceB };
+        var root = Path.Combine(Path.GetTempPath(), "ARSAS.Tests", "M4", Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(root);
+        return new Fixture(project, iedA, iedB, pointA1, pointA2, pointB, devices, root);
+    }
+
+    private static IoTestPointPlan Point(string id, string iedName, string ipAddress, string reference) => new()
+    {
+        TestPointId = id,
+        IedName = iedName,
+        IpAddress = ipAddress,
+        SignalName = id,
+        ObjectReference = reference,
+        FunctionalConstraint = "ST",
+        ExpectedOnText = "ON",
+        ExpectedOffText = "OFF",
+        ExpectedOnRaw = 1,
+        ExpectedOffRaw = 0,
+        DataType = "BOOLEAN",
+        LogicalDevice = iedName + "LD0",
+        LogicalNode = "GGIO1",
+        DataObject = "Ind",
+        DataAttribute = "stVal",
+        SourceIecReference = reference,
+        EventLogSearchReference = reference,
+        ReportDisplayReference = reference + " [ST]",
+        BindingStatus = "CID_DATASET_EXACT",
+        TestEnabled = true,
+        ImportReady = true
+    };
+
+    private static IoTestIedPlan Ied(string name, string ipAddress, params IoTestPointPlan[] points)
+    {
+        var ied = new IoTestIedPlan
+        {
+            IedName = name,
+            IpAddress = ipAddress,
+            IedRole = "Protection IED"
+        };
+        ied.TestPoints.AddRange(points);
+        return ied;
+    }
+
+    private static Iec61850MonitorDevice Device(
+        string deviceId,
+        IoTestIedPlan ied,
+        params IoTestPointPlan[] points)
+    {
+        var device = new Iec61850MonitorDevice
+        {
+            DeviceId = deviceId,
+            Name = ied.IedName,
+            SclIedName = ied.IedName,
+            IpAddress = ied.IpAddress,
+            Port = 102,
+            IsConnected = true,
+            IsMonitoring = true,
+            Status = "Monitoring"
+        };
+
+        foreach (var point in points)
+        {
+            device.Points.Add(new Iec61850MonitorPoint
+            {
+                DeviceId = deviceId,
+                DeviceName = ied.IedName,
+                IpAddress = ied.IpAddress,
+                SignalName = point.SignalName,
+                IecReference = point.ObjectReference,
+                FunctionalConstraint = "ST",
+                Value = "False",
+                Quality = "Good",
+                DeviceTimestamp = "2026-09-10T04:00:00.000Z",
+                SourceMode = "BRCB",
+                Sequence = 0,
+                Status = "Live"
+            });
+        }
+        return device;
+    }
+
+    private static IoTestSessionController NewController(
+        IoTestProject project,
+        IReadOnlyCollection<Iec61850MonitorDevice> devices,
+        string root) => new(
+            project,
+            key => devices.FirstOrDefault(device =>
+                key.Equals(device.DeviceId, StringComparison.OrdinalIgnoreCase) ||
+                key.Equals(device.Name, StringComparison.OrdinalIgnoreCase) ||
+                key.Equals(device.IpAddress, StringComparison.OrdinalIgnoreCase)),
+            action => action(),
+            root);
+
+    private sealed record Fixture(
+        IoTestProject Project,
+        IoTestIedPlan IedA,
+        IoTestIedPlan IedB,
+        IoTestPointPlan PointA1,
+        IoTestPointPlan PointA2,
+        IoTestPointPlan PointB,
+        IReadOnlyCollection<Iec61850MonitorDevice> Devices,
+        string Root);
+}
diff --git a/tests/ARSAS.Tests/IoFatM5PersistenceAuthorityRegressionTests.cs b/tests/ARSAS.Tests/IoFatM5PersistenceAuthorityRegressionTests.cs
new file mode 100644
index 000000000..401af02fb
--- /dev/null
+++ b/tests/ARSAS.Tests/IoFatM5PersistenceAuthorityRegressionTests.cs
@@ -0,0 +1,228 @@
+using System.Security.Cryptography;
+using System.Text;
+using ArIED61850Tester.Models.IoTesting;
+using ArIED61850Tester.Services.IoTesting;
+using Xunit;
+
+namespace ARSAS.Tests;
+
+public sealed class IoFatM5PersistenceAuthorityRegressionTests
+{
+    [Fact]
+    public async Task Reconcile_DeviceIdMismatch_FailsClosedWithoutRestoringEvidence()
+    {
+        var root = TempDirectory();
+        var sourcePath = await WriteSourceAsync(root, "m5-device-mismatch.xlsx", "m5-device-mismatch-source");
+        var projectsRoot = Path.Combine(root, "projects");
+        var evidenceRoot = Path.Combine(root, "evidence");
+
+        var savedIed = Ied("IED-A", "192.168.90.11", "engineering-device-old");
+        var saved = Project(Hash(sourcePath), savedIed);
+        using (var session = Session(saved, evidenceRoot))
+        {
+            var opened = await IoTestWorkspacePersistence.OpenWorkbookAsync(
+                saved, session, sourcePath, projectsRoot, evidenceRoot);
+            using (opened.Workspace)
+            {
+                CompletePass(opened.Project.Ieds.Single().TestPoints.Single());
+                opened.Workspace.SaveNow();
+            }
+        }
+
+        var evidenceFilesBefore = EvidenceFiles(evidenceRoot);
+        var current = Project(Hash(sourcePath), Ied("IED-A", "192.168.90.11", "engineering-device-new"));
+
+        var reopened = await IoTestWorkspaceBootstrapService.OpenWorkbookAsync(
+            current, sourcePath, projectsRoot, evidenceRoot, Session);
+        using (reopened.Session)
+        using (reopened.Workspace)
+        {
+            Assert.Same(current, reopened.Project);
+            var point = reopened.Project.Ieds.Single().TestPoints.Single();
+            Assert.Equal(IoTestPointState.NotStarted, point.Runtime.State);
+            Assert.Null(point.Runtime.OnEvidence);
+            Assert.Null(point.Runtime.OffEvidence);
+        }
+
+        Assert.Equal(evidenceFilesBefore, EvidenceFiles(evidenceRoot));
+    }
+
+    [Fact]
+    public async Task Reconcile_MatchingDeviceId_RestoresExistingEvidenceWithoutCreatingNewEvidenceFiles()
+    {
+        var root = TempDirectory();
+        var sourcePath = await WriteSourceAsync(root, "m5-device-match.xlsx", "m5-device-match-source");
+        var projectsRoot = Path.Combine(root, "projects");
+        var evidenceRoot = Path.Combine(root, "evidence");
+        const string deviceId = "engineering-device-ied-a";
+
+        var saved = Project(Hash(sourcePath), Ied("IED-A", "192.168.90.21", deviceId));
+        using (var session = Session(saved, evidenceRoot))
+        {
+            var opened = await IoTestWorkspacePersistence.OpenWorkbookAsync(
+                saved, session, sourcePath, projectsRoot, evidenceRoot);
+            using (opened.Workspace)
+            {
+                CompletePass(opened.Project.Ieds.Single().TestPoints.Single());
+                opened.Workspace.SaveNow();
+            }
+        }
+
+        var evidenceFilesBefore = EvidenceFiles(evidenceRoot);
+        var current = Project(Hash(sourcePath), Ied("IED-A", "192.168.90.21", deviceId));
+
+        var reopened = await IoTestWorkspaceBootstrapService.OpenWorkbookAsync(
+            current, sourcePath, projectsRoot, evidenceRoot, Session);
+        using (reopened.Session)
+        using (reopened.Workspace)
+        {
+            Assert.Same(current, reopened.Project);
+            Assert.True(reopened.RestoredProgress);
+            var point = reopened.Project.Ieds.Single().TestPoints.Single();
+            Assert.Equal(IoTestPointState.Passed, point.Runtime.State);
+            Assert.NotNull(point.Runtime.OnEvidence);
+            Assert.NotNull(point.Runtime.OffEvidence);
+        }
+
+        // Background/bootstrap reconciliation may adopt already-persisted evidence into the
+        // fresh Engineering plan, but it is not an evidence-producing operation. Only an
+        // explicit FAT session/capture path is allowed to create journal evidence.
+        Assert.Equal(evidenceFilesBefore, EvidenceFiles(evidenceRoot));
+    }
+
+    [Fact]
+    public async Task Reconcile_LegacyEmptyDeviceId_RemainsCompatibleWithExactIecEndpoint()
+    {
+        var root = TempDirectory();
+        var sourcePath = await WriteSourceAsync(root, "m5-legacy-device.xlsx", "m5-legacy-device-source");
+        var projectsRoot = Path.Combine(root, "projects");
+        var evidenceRoot = Path.Combine(root, "evidence");
+
+        // Empty DeviceId represents historical snapshots created before the M5 authority key.
+        var saved = Project(Hash(sourcePath), Ied("IED-A", "192.168.90.31", deviceId: null));
+        using (var session = Session(saved, evidenceRoot))
+        {
+            var opened = await IoTestWorkspacePersistence.OpenWorkbookAsync(
+                saved, session, sourcePath, projectsRoot, evidenceRoot);
+            using (opened.Workspace)
+            {
+                CompletePass(opened.Project.Ieds.Single().TestPoints.Single());
+                opened.Workspace.SaveNow();
+            }
+        }
+
+        var current = Project(Hash(sourcePath), Ied("IED-A", "192.168.90.31", "engineering-device-now-known"));
+        var reopened = await IoTestWorkspaceBootstrapService.OpenWorkbookAsync(
+            current, sourcePath, projectsRoot, evidenceRoot, Session);
+        using (reopened.Session)
+        using (reopened.Workspace)
+        {
+            var point = reopened.Project.Ieds.Single().TestPoints.Single();
+            Assert.Equal(IoTestPointState.Passed, point.Runtime.State);
+            Assert.NotNull(point.Runtime.OnEvidence);
+            Assert.NotNull(point.Runtime.OffEvidence);
+        }
+    }
+
+    private static IoTestProject Project(string sourceHash, IoTestIedPlan ied)
+    {
+        var project = new IoTestProject
+        {
+            ProjectId = "M5-PERSISTENCE-AUTHORITY",
+            SchemaVersion = "ARSAS-FAT-IO-1.0",
+            ProjectName = "M5 persistence authority",
+            SourceWorkbookName = "m5.xlsx",
+            SourceWorkbookSha256 = sourceHash
+        };
+        project.Ieds.Add(ied);
+        return project;
+    }
+
+    private static IoTestIedPlan Ied(string name, string ip, string? deviceId)
+    {
+        var ied = new IoTestIedPlan
+        {
+            IedName = name,
+            IpAddress = ip,
+            IedRole = "Protection IED",
+            Location = "FAT bench"
+        };
+        if (!string.IsNullOrWhiteSpace(deviceId))
+            ied.ApplyLiveDeviceBinding(deviceId, "Engineering identity bound");
+        ied.TestPoints.Add(Point("TP-001", name, ip, $"{name}LD0/GGIO1.Ind1.stVal"));
+        return ied;
+    }
+
+    private static IoTestPointPlan Point(string id, string iedName, string ip, string objectReference)
+        => new()
+        {
+            TestPointId = id,
+            IedName = iedName,
+            IpAddress = ip,
+            SignalName = "Binary indication",
+            ObjectReference = objectReference,
+            FunctionalConstraint = "ST",
+            ExpectedOnText = "Active",
+            ExpectedOffText = "InActive",
+            ExpectedOnRaw = 1,
+            ExpectedOffRaw = 0,
+            DataType = "SDI",
+            SourceIecReference = objectReference,
+            EventLogSearchReference = objectReference,
+            SignalKind = FatSignalKind.Discrete,
+            CaptureMode = FatCaptureMode.AutomaticTransition,
+            ImportReady = true,
+            BindingStatus = "CID_DATASET_EXACT"
+        };
+
+    private static void CompletePass(IoTestPointPlan point)
+    {
+        var evaluator = new IoTestTransitionEvaluator();
+        evaluator.StartAttempt(point, Observation(false, 1));
+        evaluator.Observe(point, Observation(true, 2));
+        evaluator.Observe(point, Observation(false, 3));
+        Assert.Equal(IoTestPointState.Passed, point.Runtime.State);
+    }
+
+    private static IoTestObservation Observation(bool state, long sequence)
+    {
+        var timestamp = new DateTimeOffset(2026, 9, 10, 3, 0, 0, TimeSpan.Zero).AddMilliseconds(sequence * 100);
+        return new IoTestObservation(
+            state,
+            state ? "True" : "False",
+            timestamp,
+            timestamp.AddMilliseconds(-3),
+            "Good",
+            "BRCB",
+            sequence,
+            1);
+    }
+
+    private static IoTestSessionController Session(IoTestProject project, string evidenceRoot)
+        => new(project, _ => null, action => action(), evidenceRoot);
+
+    private static async Task<string> WriteSourceAsync(string root, string fileName, string content)
+    {
+        var path = Path.Combine(root, fileName);
+        await File.WriteAllBytesAsync(path, Encoding.UTF8.GetBytes(content));
+        return path;
+    }
+
+    private static string[] EvidenceFiles(string evidenceRoot)
+        => !Directory.Exists(evidenceRoot)
+            ? Array.Empty<string>()
+            : Directory.EnumerateFiles(evidenceRoot, "*", SearchOption.AllDirectories)
+                .Select(path => Path.GetRelativePath(evidenceRoot, path))
+                .OrderBy(path => path, StringComparer.Ordinal)
+                .ToArray();
+
+    private static string Hash(string path)
+        => Convert.ToHexString(SHA256.HashData(File.ReadAllBytes(path))).ToLowerInvariant();
+
+    private static string TempDirectory()
+    {
+        var path = Path.Combine(Path.GetTempPath(), "ARSAS.Tests", Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(path);
+        return path;
+    }
+}
diff --git a/tests/ARSAS.Tests/IoFatPhaseBPerIedRestoreRegressionTests.cs b/tests/ARSAS.Tests/IoFatPhaseBPerIedRestoreRegressionTests.cs
index 4b263b5c0..6e2afdcad 100644
--- a/tests/ARSAS.Tests/IoFatPhaseBPerIedRestoreRegressionTests.cs
+++ b/tests/ARSAS.Tests/IoFatPhaseBPerIedRestoreRegressionTests.cs
@@ -134,6 +134,10 @@ public async Task Reopen_RequiresTechnicalKeyAndIpForIedOwnership()
         using (reopened.Session)
         using (reopened.Workspace)
         {
+            // Bootstrap must keep the freshly imported Engineering plan as authority. A
+            // persistence candidate may contribute progress only after IED ownership matches;
+            // it must never replace the current project wholesale.
+            Assert.Same(current, reopened.Project);
             var point = reopened.Project.Ieds.Single().TestPoints.Single();
             Assert.Equal(IoTestPointState.NotStarted, point.Runtime.State);
             Assert.Null(point.Runtime.OnEvidence);
diff --git a/tests/ARSAS.Tests/IoTestingUiContractTests.cs b/tests/ARSAS.Tests/IoTestingUiContractTests.cs
index 1e430db40..4ab3cd33b 100644
--- a/tests/ARSAS.Tests/IoTestingUiContractTests.cs
+++ b/tests/ARSAS.Tests/IoTestingUiContractTests.cs
@@ -195,7 +195,8 @@ public void IoTestingWindow_UsesRelayTimestampEvidenceAndPremiumGridDensity()
         Assert.Contains(
             document.Descendants(presentation + "Setter"),
             setter => (string?)setter.Attribute("Property") == "MinHeight" &&
-                      (string?)setter.Attribute("Value") == "48");
+                      (string?)setter.Attribute("Value") == "40");
+        Assert.StartsWith("Inter", (string?)document.Root?.Attribute("FontFamily"), StringComparison.Ordinal);
         Assert.Contains("RelayIcon", text, StringComparison.Ordinal);
         Assert.Contains("CardStateText", text, StringComparison.Ordinal);
         Assert.Contains("✔ PASS", text, StringComparison.Ordinal);
@@ -270,11 +271,13 @@ public void IoTestingWindow_UsesBalancedInitialGridWidthsAndCenteredRelayHeaders
         var onTime = FindColumn(document, presentation, "ON · RELAY TIME");
         var offTime = FindColumn(document, presentation, "OFF · RELAY TIME");
 
-        Assert.Equal("1.55*", (string?)signal.Attribute("Width"));
-        Assert.Equal("1.85*", (string?)reference.Attribute("Width"));
-        Assert.Equal("136", (string?)acquisition.Attribute("Width"));
-        Assert.Equal("168", (string?)onTime.Attribute("Width"));
-        Assert.Equal("168", (string?)offTime.Attribute("Width"));
+        Assert.Equal("1.35*", (string?)signal.Attribute("Width"));
+        Assert.Equal("2.25*", (string?)reference.Attribute("Width"));
+        Assert.Equal("290", (string?)reference.Attribute("MinWidth"));
+        Assert.Equal("480", (string?)reference.Attribute("MaxWidth"));
+        Assert.Equal("122", (string?)acquisition.Attribute("Width"));
+        Assert.Equal("154", (string?)onTime.Attribute("Width"));
+        Assert.Equal("154", (string?)offTime.Attribute("Width"));
         Assert.Equal("{StaticResource CenteredFatGridHeader}", (string?)onTime.Attribute("HeaderStyle"));
         Assert.Equal("{StaticResource CenteredFatGridHeader}", (string?)offTime.Attribute("HeaderStyle"));
     }
diff --git a/tests/ARSAS.Tests/MainWindowTopBarLayoutRegressionTests.cs b/tests/ARSAS.Tests/MainWindowTopBarLayoutRegressionTests.cs
index 67a83f8a4..7a2a4aca7 100644
--- a/tests/ARSAS.Tests/MainWindowTopBarLayoutRegressionTests.cs
+++ b/tests/ARSAS.Tests/MainWindowTopBarLayoutRegressionTests.cs
@@ -3,7 +3,7 @@ namespace ARSAS.Tests;
 public sealed class MainWindowTopBarLayoutRegressionTests
 {
     [Fact]
-    public void NavigationLayout_UsesResponsiveWidthsAndConsistentSixDestinationLabels()
+    public void NavigationLayout_UsesResponsiveWidthsAndConsistentSevenDestinationGeometry()
     {
         var source = File.ReadAllText(FindRepoFile("MainWindow.NavigationLayoutFix.cs"));
 
@@ -14,20 +14,22 @@ public void NavigationLayout_UsesResponsiveWidthsAndConsistentSixDestinationLabe
         Assert.Contains("\"Alarm\"", source, StringComparison.Ordinal);
         Assert.DoesNotContain("\"Alarm Annunciator\"", source, StringComparison.Ordinal);
         Assert.Contains("\"GOOSE Subscriber\"", source, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton", source, StringComparison.Ordinal);
         Assert.Contains("var labels = wide ? FullLabels : CompactLabels", source, StringComparison.Ordinal);
     }
 
     [Fact]
-    public void SelectionPill_IsDerivedFromActualSixCellNavWidth_NotFixed150Pixels()
+    public void SelectionPill_IsDerivedFromActualSevenCellNavWidth_NotFixed150Pixels()
     {
         var source = File.ReadAllText(FindRepoFile("MainWindow.NavigationLayoutFix.cs"));
 
-        Assert.Contains("var cellWidth = contentWidth / 6d", source, StringComparison.Ordinal);
-        Assert.Contains("Math.Clamp(tabs.SelectedIndex, 0, 5) * cellWidth", source, StringComparison.Ordinal);
+        Assert.Contains("var cellWidth = contentWidth / 7d", source, StringComparison.Ordinal);
+        Assert.Contains("Math.Clamp(tabs.SelectedIndex, 0, 6) * cellWidth", source, StringComparison.Ordinal);
         Assert.Contains("pill.Width = Math.Max(1d, cellWidth - 2d)", source, StringComparison.Ordinal);
         Assert.Contains("FindName(\"WorkflowPillTranslate\") as TranslateTransform", source, StringComparison.Ordinal);
         Assert.Contains("pill.RenderTransform is TransformGroup", source, StringComparison.Ordinal);
         Assert.DoesNotContain("* 150d", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("contentWidth / 6d", source, StringComparison.Ordinal);
         Assert.DoesNotContain("contentWidth / 5d", source, StringComparison.Ordinal);
     }
 
@@ -41,6 +43,7 @@ public void ResponsiveCorrection_RunsAfterSelectionResizeAndRepeatedNavClick()
         Assert.Contains("Button.ClickEvent", source, StringComparison.Ordinal);
         Assert.Contains("OnMainWindowButtonClick", source, StringComparison.Ordinal);
         Assert.Contains("NavAlarmButton", source, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton", source, StringComparison.Ordinal);
         Assert.Contains("A repeated click on the already-selected tab", source, StringComparison.Ordinal);
         Assert.Contains("DispatcherPriority.Loaded", source, StringComparison.Ordinal);
         Assert.Contains("DispatcherPriority.ContextIdle", source, StringComparison.Ordinal);
diff --git a/tests/ARSAS.Tests/ProductionFatEngineeringTabRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatEngineeringTabRegressionTests.cs
index 46509c39d..9a80fe0fa 100644
--- a/tests/ARSAS.Tests/ProductionFatEngineeringTabRegressionTests.cs
+++ b/tests/ARSAS.Tests/ProductionFatEngineeringTabRegressionTests.cs
@@ -21,12 +21,13 @@ public void ProductionFatTab_AutoBootstrapsFromSelectedEngineeringStaticDataSet(
     {
         var source = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatEngineeringBootstrap.cs"));
 
-        Assert.Contains("MainTabs.SelectedIndex != NativeFatWorkspaceIndex", source, StringComparison.Ordinal);
+        Assert.Contains("QueueProductionFatEngineeringBootstrap();", source, StringComparison.Ordinal);
         Assert.Contains("selected?.SclWorkspace", source, StringComparison.Ordinal);
         Assert.Contains("DesignModel.DataSets.Sum", source, StringComparison.Ordinal);
         Assert.Contains("IoFatEngineeringWorkspaceProjectionService.BuildAsync", source, StringComparison.Ordinal);
         Assert.Contains("AdoptEngineeringRuntimeWorkspaces", source, StringComparison.Ordinal);
-        Assert.Contains("IoTestWorkspaceBootstrapService.OpenSourcesAsync", source, StringComparison.Ordinal);
+        Assert.Contains("IoTestWorkspaceBootstrapService.OpenDescribedSourcesAsync", source, StringComparison.Ordinal);
+        Assert.Contains("projection.DescribedSources", source, StringComparison.Ordinal);
         Assert.Contains("SynchronizeImportedSclFatWithEngineering", source, StringComparison.Ordinal);
         Assert.Contains("ShowIoTestingWorkspaceAsync", source, StringComparison.Ordinal);
         Assert.Contains("no SCL re-import", source, StringComparison.Ordinal);
@@ -44,31 +45,82 @@ public void ProductionFatTab_RegistersExactEngineeringRuntimeWorkspacesForShared
     }
 
     [Fact]
-    public void SeventhFatDestination_UsesSameSelectionPillAndActiveForegroundContract()
+    public void SeventhFatDestination_IsCanonicalMainWindowSiblingWithOneNavigationOwner()
     {
-        var source = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatNavigationParity.cs"));
-
-        Assert.Contains("NavNativeFatButton", source, StringComparison.Ordinal);
-        Assert.Contains("buttons.Length", source, StringComparison.Ordinal);
-        Assert.Contains("Math.Clamp(tabs.SelectedIndex, 0, 6)", source, StringComparison.Ordinal);
-        Assert.Contains("var cellWidth = contentWidth / 7d", source, StringComparison.Ordinal);
-        Assert.Contains("pill.Width = Math.Max(1d, cellWidth - 2d)", source, StringComparison.Ordinal);
-        Assert.Contains("button.Foreground = index == selectedIndex ? Brushes.White : muted", source, StringComparison.Ordinal);
-        Assert.Contains("DispatcherPriority.ApplicationIdle", source, StringComparison.Ordinal);
+        var xaml = File.ReadAllText(FindRepoFile("MainWindow.xaml"));
+        var mainSource = File.ReadAllText(FindRepoFile("MainWindow.xaml.cs"));
+        var bridgeSource = File.ReadAllText(FindRepoFile("MainWindow.NativeFatWorkspace.cs"));
+        var productionSource = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatTab.cs"));
+        var repoRoot = FindRepoRoot();
+
+        Assert.Contains("x:Name=\"NavNativeFatButton\" Grid.Column=\"6\" Content=\"FAT\" Tag=\"6\" Click=\"NavButton_Click\" Style=\"{StaticResource SegmentedNavButton}\"", xaml, StringComparison.Ordinal);
+        Assert.Contains("Grid.ColumnSpan=\"7\"", xaml, StringComparison.Ordinal);
+        Assert.Contains("x:Name=\"NativeFatTab\" Header=\"FAT\"", xaml, StringComparison.Ordinal);
+
+        Assert.Contains("Math.Clamp(index, 0, NativeFatWorkspaceIndex)", mainSource, StringComparison.Ordinal);
+        Assert.Contains("var cellWidth = contentWidth / 7d", mainSource, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton", mainSource, StringComparison.Ordinal);
+
+        Assert.Contains("private const int NativeFatWorkspaceIndex = 6", bridgeSource, StringComparison.Ordinal);
+        Assert.Contains("QueueNativeFatNavigationGeometry", bridgeSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatTab", bridgeSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatNavButton", bridgeSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("BuildNativeFatWorkspaceContent", bridgeSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("DataGrid", bridgeSource, StringComparison.Ordinal);
+
+        Assert.Contains("NativeFatTab.Content = BuildProductionFatPermanentHost();", productionSource, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton.ToolTip", productionSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("ProductionFatNavButton_Click", productionSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatInstalled", productionSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatTab", productionSource, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatNavButton", productionSource, StringComparison.Ordinal);
+
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.ProductionFatNavigationParity.cs")),
+            "Do not reintroduce a second ApplicationIdle navigation parity owner. MainWindow is the seven-slot navigation authority.");
+    }
+
+    [Fact]
+    public void ExistingWorkspaceSelectionSideEffects_RemainProtectedWhileAddingFat()
+    {
+        var source = File.ReadAllText(FindRepoFile("MainWindow.xaml.cs"));
+
+        Assert.Contains("device.ClearUnreadEvents()", source, StringComparison.Ordinal);
+        Assert.Contains("ActivateGooseSubscriberWorkspace()", source, StringComparison.Ordinal);
+        Assert.Contains("ClearDiagnosticAlert()", source, StringComparison.Ordinal);
+        Assert.Contains("UpdateNavigationVisuals(MainTabs.SelectedIndex, animate: true)", source, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void ProductionFatSafetyBoundary_KeepsEngineeringAuthorityAndStrictPreflightCoverage()
+    {
+        var bootstrapSource = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatEngineeringBootstrap.cs"));
+        var fieldRegressionSource = File.ReadAllText(FindRepoFile("tests/ARSAS.Tests/ProductionFatP0FieldRegressionTests.cs"));
+
+        Assert.Contains("RetireManualWorkspaceRowsForStaticDataSetMode", bootstrapSource, StringComparison.Ordinal);
+        Assert.Contains("AutomaticStaticDataSetScope_RetiresManualAliasBeforeSessionPreflight", fieldRegressionSource, StringComparison.Ordinal);
+        Assert.Contains("IoTestSessionPreflight.Validate", fieldRegressionSource, StringComparison.Ordinal);
+        Assert.Contains("multiple enabled test points", fieldRegressionSource, StringComparison.OrdinalIgnoreCase);
     }
 
     private static string FindRepoFile(string relativePath)
+        => Path.Combine(FindRepoRoot(), relativePath);
+
+    private static string FindRepoRoot()
     {
         DirectoryInfo? directory = new(AppContext.BaseDirectory);
         while (directory != null)
         {
-            var candidate = Path.Combine(directory.FullName, relativePath);
-            if (File.Exists(candidate))
-                return candidate;
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+            {
+                return directory.FullName;
+            }
+
             directory = directory.Parent;
         }
 
-        throw new FileNotFoundException(
-            $"Could not locate repository file '{relativePath}' from '{AppContext.BaseDirectory}'.");
+        throw new DirectoryNotFoundException(
+            $"Could not locate repository root from '{AppContext.BaseDirectory}'.");
     }
 }
diff --git a/tests/ARSAS.Tests/ProductionFatFieldPerformanceRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatFieldPerformanceRegressionTests.cs
new file mode 100644
index 000000000..f6fe6b3c4
--- /dev/null
+++ b/tests/ARSAS.Tests/ProductionFatFieldPerformanceRegressionTests.cs
@@ -0,0 +1,46 @@
+namespace ARSAS.Tests;
+
+public sealed class ProductionFatFieldPerformanceRegressionTests
+{
+    [Fact]
+    public void EmbeddedStaticDataSetFat_ReleasesLegacy250msPollingOverride()
+    {
+        var source = Read("MainWindow.ProductionFatFieldPerformance.cs");
+
+        Assert.Contains("ReleaseLegacyPollingOverrideForStaticEmbeddedFat", source, StringComparison.Ordinal);
+        Assert.Contains("IsSharedStaticDataSetAuthority", source, StringComparison.Ordinal);
+        Assert.Contains("PollingIntervalMs = _pollingIntervalBeforeIoFat.Value;", source, StringComparison.Ordinal);
+        Assert.Contains("_pollingIntervalBeforeIoFat = null;", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("UseHybrid", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("MMS polling", source, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void ManualOrWorkbookFat_DoesNotLoseLegacyPollingCadence()
+    {
+        var source = Read("MainWindow.ProductionFatFieldPerformance.cs");
+
+        Assert.Contains(
+            "liveDevices.Any(device => !IsSharedStaticDataSetAuthority(device!))",
+            source,
+            StringComparison.Ordinal);
+        Assert.Contains("return;", source, StringComparison.Ordinal);
+    }
+
+    private static string Read(string relativePath)
+        => File.ReadAllText(Path.Combine(FindRepoRoot(), relativePath)).Replace("\r\n", "\n", StringComparison.Ordinal);
+
+    private static string FindRepoRoot()
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+                return directory.FullName;
+            directory = directory.Parent;
+        }
+
+        throw new DirectoryNotFoundException("Could not locate repository root.");
+    }
+}
diff --git a/tests/ARSAS.Tests/ProductionFatM2PermanentHostRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatM2PermanentHostRegressionTests.cs
new file mode 100644
index 000000000..5f113b828
--- /dev/null
+++ b/tests/ARSAS.Tests/ProductionFatM2PermanentHostRegressionTests.cs
@@ -0,0 +1,148 @@
+namespace ARSAS.Tests;
+
+public sealed class ProductionFatM2PermanentHostRegressionTests
+{
+    [Fact]
+    public void FatTab_IsPermanentHost_NotLauncherOrAlternateOpenSclWorkflow()
+    {
+        var source = Read("MainWindow.ProductionFatTab.cs");
+
+        Assert.Contains("BuildProductionFatPermanentHost()", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("BuildProductionFatLauncher", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("Open SCL for FAT", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("Open ARSAS Project", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("OpenSclFatTesting_Click", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("OpenIoListPackage_Click", source, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void EngineeringStaticDataSet_FatBootstrapIsNavigationGated()
+    {
+        var bootstrap = Read("MainWindow.ProductionFatEngineeringBootstrap.cs");
+
+        Assert.DoesNotContain(
+            "window.QueueProductionFatEngineeringBootstrap();\n    }\n\n    private void ProductionFatEngineeringBootstrap_SelectionChanged",
+            bootstrap,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "e.PropertyName != nameof(SelectedDevice) || MainTabs.SelectedIndex != NativeFatWorkspaceIndex",
+            bootstrap,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "!_productionFatEngineeringBootstrapInstalled || MainTabs.SelectedIndex != NativeFatWorkspaceIndex",
+            bootstrap,
+            StringComparison.Ordinal);
+        Assert.Contains(
+            "_productionFatEngineeringBootstrapBusy ||\n            MainTabs.SelectedIndex != NativeFatWorkspaceIndex",
+            bootstrap,
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void AutomaticFatBootstrap_PreservesStaticReportOnlyAuthority()
+    {
+        var bootstrap = Read("MainWindow.ProductionFatEngineeringBootstrap.cs");
+        var authority = Read("MainWindow.SharedStaticDataSetAuthority.cs");
+
+        Assert.Contains("PreserveSharedStaticDataSetAuthority(device);", bootstrap, StringComparison.Ordinal);
+        Assert.DoesNotContain("MarkSharedSelectionAuthority(device);", bootstrap, StringComparison.Ordinal);
+        Assert.Contains("Iec61850MonitoringModeRegistry.UseStaticDataSetReportOnly(device);", authority, StringComparison.Ordinal);
+        Assert.DoesNotContain("UseHybrid", authority, StringComparison.Ordinal);
+        Assert.Contains("_sharedSclStaticDataSetAuthorityDeviceIds.Add(device.DeviceId);", authority, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void EmbeddedDonor_IsPreparedBeforeShow_AndNormalizesIllegalMaximizedState()
+    {
+        var launch = Read("MainWindow.IoTesting.cs");
+        var host = Read("IoListTestingWindow.EmbeddedEngineeringHost.cs");
+        var tab = Read("MainWindow.ProductionFatTab.cs");
+
+        var prepare = launch.IndexOf("window.PrepareForEmbeddedEngineeringHost();", StringComparison.Ordinal);
+        var show = launch.IndexOf("window.Show();", StringComparison.Ordinal);
+        Assert.True(prepare >= 0 && show > prepare, "Donor visibility must be prepared before Window.Show().");
+        Assert.Contains("if (!ProductionFatTabReady)\n            Hide();", launch, StringComparison.Ordinal);
+
+        var normal = host.IndexOf("WindowState = WindowState.Normal;", StringComparison.Ordinal);
+        var nonActivating = host.IndexOf("ShowActivated = false;", StringComparison.Ordinal);
+        Assert.True(normal >= 0 && nonActivating > normal,
+            "The hidden donor must leave Maximized state before ShowActivated=false is used.");
+        Assert.Contains("Left = -32000d", host, StringComparison.Ordinal);
+        Assert.Contains("Opacity = 0d", host, StringComparison.Ordinal);
+        Assert.Contains("ShowInTaskbar = false", host, StringComparison.Ordinal);
+
+        Assert.DoesNotContain("MainTabs.SelectedIndex = NativeFatWorkspaceIndex", tab, StringComparison.Ordinal);
+        Assert.DoesNotContain("Activate();", tab, StringComparison.Ordinal);
+        Assert.DoesNotContain("if (!IsVisible)\n            Show();", tab, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void HiddenNonActivatingWpfDonor_CanShowAfterStateNormalization()
+    {
+        if (!OperatingSystem.IsWindows())
+            return;
+
+        Exception? failure = null;
+        var completed = false;
+        var thread = new Thread(() =>
+        {
+            try
+            {
+                var window = new System.Windows.Window
+                {
+                    WindowState = System.Windows.WindowState.Normal,
+                    ShowActivated = false,
+                    ShowInTaskbar = false,
+                    Opacity = 0d,
+                    WindowStartupLocation = System.Windows.WindowStartupLocation.Manual,
+                    Left = -32000d,
+                    Top = -32000d
+                };
+
+                window.Show();
+                window.Close();
+                completed = true;
+            }
+            catch (Exception ex)
+            {
+                failure = ex;
+            }
+        });
+
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+        Assert.True(thread.Join(TimeSpan.FromSeconds(8)), "WPF donor Show/Close timed out.");
+        Assert.Null(failure);
+        Assert.True(completed);
+    }
+
+    [Fact]
+    public void EmbeddedSurface_RemainsExactProductionGridAndPreviewSource()
+    {
+        var host = Read("IoListTestingWindow.EmbeddedEngineeringHost.cs");
+
+        Assert.Contains("InstallFatV2WorkspaceUx();", host, StringComparison.Ordinal);
+        Assert.Contains("InstallPerIedPrintPreview();", host, StringComparison.Ordinal);
+        Assert.Contains("DetachProductionFatCentralWorkspace()", host, StringComparison.Ordinal);
+        Assert.Contains("owner.MountProductionFatWorkspace(this, surface)", host, StringComparison.Ordinal);
+        Assert.Contains("throw new InvalidOperationException(\"Production FAT center could not be detached", host, StringComparison.Ordinal);
+        Assert.Contains("WindowState = WindowState.Maximized;", host, StringComparison.Ordinal);
+    }
+
+    private static string Read(string relativePath)
+        => File.ReadAllText(Path.Combine(FindRepoRoot(), relativePath)).Replace("\r\n", "\n", StringComparison.Ordinal);
+
+    private static string FindRepoRoot()
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+                return directory.FullName;
+            directory = directory.Parent;
+        }
+
+        throw new DirectoryNotFoundException("Could not locate repository root.");
+    }
+}
diff --git a/tests/ARSAS.Tests/ProductionFatM3SelectedIedBindingRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatM3SelectedIedBindingRegressionTests.cs
new file mode 100644
index 000000000..c8d50ce60
--- /dev/null
+++ b/tests/ARSAS.Tests/ProductionFatM3SelectedIedBindingRegressionTests.cs
@@ -0,0 +1,104 @@
+namespace ARSAS.Tests;
+
+public sealed class ProductionFatM3SelectedIedBindingRegressionTests
+{
+    [Fact]
+    public void Start_LatchesCaptureTarget_WithoutTakingOverViewedContext()
+    {
+        var window = Read("IoListTestingWindow.xaml.cs");
+        var coordinator = Read("Services/IoTesting/IoTestMultiSessionCoordinator.cs");
+
+        var requested = window.IndexOf("var requestedIed = SelectedIed;", StringComparison.Ordinal);
+        var latch = window.IndexOf("IoFatProductionControllerAdapter.LatchStartTarget(Project, requestedIed)", requested, StringComparison.Ordinal);
+        var selected = window.IndexOf("var selectedIed = captureLease.Ied;", latch, StringComparison.Ordinal);
+        var prepare = window.IndexOf("await engineeringWindow.PrepareIoTestIedForFatAsync(", selected, StringComparison.Ordinal);
+        var start = window.IndexOf("IoFatProductionControllerAdapter.StartLatched(Project, Session, captureLease)", prepare, StringComparison.Ordinal);
+
+        Assert.True(requested >= 0, "Start FAT must read the currently viewed IED before asynchronous preparation.");
+        Assert.True(latch > requested, "Production FAT must freeze the IED and exact capture scope before asynchronous preparation.");
+        Assert.True(selected > latch, "The preparation target must come from the frozen production capture lease.");
+        Assert.True(prepare > selected, "Engineering acquisition must prepare the latched target.");
+        Assert.True(start > prepare, "The production session must start from the same frozen lease after preparation.");
+
+        var startBody = MethodBody(
+            coordinator,
+            "public IoTestSessionActionResult Start(\n        IoTestIedPlan? ied,\n        IReadOnlyCollection<IoTestPointPlan>? captureScope)",
+            "public IoTestSessionActionResult Pause");
+        Assert.DoesNotContain("SelectContext(ied)", startBody, StringComparison.Ordinal);
+        Assert.Contains("GetOrCreateController(ied)", startBody, StringComparison.Ordinal);
+        Assert.Contains("controller.Start(ied, captureScope)", startBody, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void EngineeringExplorer_RemainsViewedIedAuthority_WhileSessionsMayRun()
+    {
+        var owner = Read("MainWindow.ProductionFatTab.cs");
+        var embedded = Read("IoListTestingWindow.EmbeddedEngineeringHost.cs");
+        var window = Read("IoListTestingWindow.xaml.cs");
+
+        var propertyHandler = MethodBody(
+            owner,
+            "private void ProductionFat_MainWindowPropertyChanged",
+            "private void SynchronizeProductionFatSelectedIed()");
+        Assert.Contains("e.PropertyName == nameof(SelectedDevice)", propertyHandler, StringComparison.Ordinal);
+        Assert.Contains("SynchronizeProductionFatSelectedIed();", propertyHandler, StringComparison.Ordinal);
+        Assert.DoesNotContain("HasRunningSession", propertyHandler, StringComparison.Ordinal);
+
+        var selectDevice = MethodBody(
+            embedded,
+            "internal void SelectEngineeringDeviceForEmbeddedFat",
+            "internal void NotifyEmbeddedHostActivated");
+        Assert.Contains("SelectedIed = null;", selectDevice, StringComparison.Ordinal);
+        Assert.Contains("SelectedIed = match;", selectDevice, StringComparison.Ordinal);
+        Assert.Contains("Session.SelectContext", window, StringComparison.Ordinal);
+        Assert.Contains("public bool CanSelectIed => true;", window, StringComparison.Ordinal);
+
+        var deviceId = selectDevice.IndexOf("device.DeviceId", StringComparison.Ordinal);
+        var sclIedName = selectDevice.IndexOf("device.SclIedName", StringComparison.Ordinal);
+        var displayName = selectDevice.IndexOf("device.Name", StringComparison.Ordinal);
+        var ipAddress = selectDevice.IndexOf("device.IpAddress", StringComparison.Ordinal);
+        Assert.True(deviceId >= 0, "Engineering DeviceId must be the strongest view identity.");
+        Assert.True(sclIedName > deviceId, "SCL IED identity must follow DeviceId fallback.");
+        Assert.True(displayName > sclIedName, "Engineering display name must be weaker than SCL identity.");
+        Assert.True(ipAddress > displayName, "IP address must be the final view-binding fallback.");
+    }
+
+    [Fact]
+    public void EvidenceActions_ResolveOwningActiveController_NotCurrentView()
+    {
+        var coordinator = Read("Services/IoTesting/IoTestMultiSessionCoordinator.cs");
+
+        Assert.Contains("ResolveOwningActiveController", coordinator, StringComparison.Ordinal);
+        Assert.Contains("points.Any(ied.TestPoints.Contains)", coordinator, StringComparison.Ordinal);
+        Assert.Contains("points.Any(point => !owners[0].TestPoints.Contains(point))", coordinator, StringComparison.Ordinal);
+        Assert.Contains("_controllers.TryGetValue(owners[0], out var controller)", coordinator, StringComparison.Ordinal);
+        Assert.Contains("!controller.IsSessionActive", coordinator, StringComparison.Ordinal);
+    }
+
+    private static string MethodBody(string source, string startMarker, string nextMarker)
+    {
+        var start = source.IndexOf(startMarker, StringComparison.Ordinal);
+        Assert.True(start >= 0, $"Missing method marker: {startMarker}");
+        var end = source.IndexOf(nextMarker, start + startMarker.Length, StringComparison.Ordinal);
+        Assert.True(end > start, $"Missing following method marker: {nextMarker}");
+        return source[start..end];
+    }
+
+    private static string Read(string relativePath)
+        => File.ReadAllText(Path.Combine(FindRepoRoot(), relativePath))
+            .Replace("\r\n", "\n", StringComparison.Ordinal);
+
+    private static string FindRepoRoot()
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+                return directory.FullName;
+            directory = directory.Parent;
+        }
+
+        throw new DirectoryNotFoundException("Could not locate repository root.");
+    }
+}
diff --git a/tests/ARSAS.Tests/ProductionFatM6ReportPreviewRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatM6ReportPreviewRegressionTests.cs
new file mode 100644
index 000000000..2da47482c
--- /dev/null
+++ b/tests/ARSAS.Tests/ProductionFatM6ReportPreviewRegressionTests.cs
@@ -0,0 +1,130 @@
+namespace ARSAS.Tests;
+
+public sealed class ProductionFatM6ReportPreviewRegressionTests
+{
+    [Fact]
+    public void EmbeddedFat_UsesExactProductionPreviewInsideTheSameCenter()
+    {
+        var xaml = File.ReadAllText(FindRepoFile("IoListTestingWindow.xaml"));
+        var embedded = File.ReadAllText(FindRepoFile("IoListTestingWindow.EmbeddedEngineeringHost.cs"));
+        var preview = File.ReadAllText(FindRepoFile("IoListTestingWindow.PrintPreview.cs"));
+        var productionTab = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatTab.cs"));
+
+        Assert.Contains("x:Name=\"WorkspacePreviewToggle\"", xaml, StringComparison.Ordinal);
+        Assert.Contains("Content=\"Print Preview\"", xaml, StringComparison.Ordinal);
+        Assert.Contains("Click=\"TogglePrintPreview_Click\"", xaml, StringComparison.Ordinal);
+
+        Assert.Contains("InstallPerIedPrintPreview();", embedded, StringComparison.Ordinal);
+        Assert.Contains("_printPreviewToggle = WorkspacePreviewToggle;", embedded, StringComparison.Ordinal);
+        Assert.Contains("DetachProductionFatCentralWorkspace", embedded, StringComparison.Ordinal);
+
+        // MainWindow hosts the exact detached production center in the canonical XAML FAT tab.
+        // Explorer and Command Dock remain owned by the workstation shell; preview never
+        // replaces MainWindow itself and no retired native FAT tab authority is recreated.
+        Assert.Contains("NativeFatTab.Content = surface;", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatTab.Content = surface;", productionTab, StringComparison.Ordinal);
+        Assert.Contains("global Engineering IED Explorer and shared Command Dock remain authoritative", productionTab, StringComparison.Ordinal);
+
+        var install = ExtractMethod(preview, "private void InstallPerIedPrintPreview()");
+        Assert.Contains("_signalWorkspaceGrid = workspaceGrid.Children.OfType<DataGrid>().FirstOrDefault();", install, StringComparison.Ordinal);
+        Assert.Contains("_printPreviewHost = BuildPrintPreviewHost();", install, StringComparison.Ordinal);
+        Assert.Contains("workspaceGrid.Children.Add(_printPreviewHost);", install, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void PreviewToggle_PreservesTheExactProductionGridInstanceAndCaptureSession()
+    {
+        var source = File.ReadAllText(FindRepoFile("IoListTestingWindow.PrintPreview.cs"));
+        var toggle = ExtractMethod(source, "private void TogglePrintPreview_Click(object sender, RoutedEventArgs e)");
+
+        Assert.Contains("_signalWorkspaceGrid.Visibility = _printPreviewActive ? Visibility.Collapsed : Visibility.Visible;", toggle, StringComparison.Ordinal);
+        Assert.Contains("_printPreviewHost.Visibility = _printPreviewActive ? Visibility.Visible : Visibility.Collapsed;", toggle, StringComparison.Ordinal);
+        Assert.Contains("RefreshPrintPreview();", toggle, StringComparison.Ordinal);
+
+        // M6 is an in-place view switch. Recreating the grid would lose scroll/selection,
+        // while touching session lifecycle here could redirect or interrupt an active capture.
+        Assert.DoesNotContain("new DataGrid", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Session.Stop", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Session.Pause", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Session.Resume", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Cancel", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Dispose", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("Show()", toggle, StringComparison.Ordinal);
+        Assert.DoesNotContain("ShowDialog", toggle, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void PreviewAndNativePdf_ShareOneReportLayoutAuthority()
+    {
+        var preview = File.ReadAllText(FindRepoFile("IoListTestingWindow.PrintPreview.cs"));
+        var documentBuilder = File.ReadAllText(FindRepoFile("Services/IoTesting/IoFatReportPreviewDocumentBuilder.cs"));
+        var pdf = File.ReadAllText(FindRepoFile("Services/IoTesting/IoFatPdfReportService.cs"));
+
+        var refresh = ExtractMethod(preview, "private void RefreshPrintPreview()");
+        Assert.Contains("IoFatReportPreviewService.CreateIedScopedProject(Project, SelectedIed)", refresh, StringComparison.Ordinal);
+        Assert.Contains("IoFatReportPreviewDocumentBuilder.Build(scopedProject, draft)", refresh, StringComparison.Ordinal);
+        Assert.Contains("Session.IsSessionActive && ReferenceEquals(Session.ActiveIed, SelectedIed)", refresh, StringComparison.Ordinal);
+
+        Assert.Contains("IoFatPdfReportService.BuildLayout(project, generatedAt, draft)", documentBuilder, StringComparison.Ordinal);
+        Assert.Contains("var layout = BuildLayout(project, generatedAt, draft: false);", pdf, StringComparison.Ordinal);
+        Assert.Contains("IoFatNativePdfWriter.Build(layout, project)", pdf, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void ActivePreview_TracksViewedIedAndEvidenceWithoutStoppingCapture()
+    {
+        var source = File.ReadAllText(FindRepoFile("IoListTestingWindow.PrintPreview.cs"));
+
+        Assert.Contains("_printPreviewActive && e.PropertyName == nameof(SelectedIed)", source, StringComparison.Ordinal);
+        Assert.Contains("e.PropertyName is nameof(Session.State) or nameof(Session.EvidenceRecordCount)", source, StringComparison.Ordinal);
+        Assert.Contains("Dispatcher.BeginInvoke(DispatcherPriority.Background, RefreshPrintPreview)", source, StringComparison.Ordinal);
+    }
+
+    private static string ExtractMethod(string source, string signature)
+    {
+        var start = source.IndexOf(signature, StringComparison.Ordinal);
+        Assert.True(start >= 0, $"Could not find method '{signature}'.");
+
+        var openBrace = source.IndexOf('{', start);
+        Assert.True(openBrace >= 0, $"Could not find opening brace for '{signature}'.");
+
+        var depth = 0;
+        for (var index = openBrace; index < source.Length; index++)
+        {
+            switch (source[index])
+            {
+                case '{':
+                    depth++;
+                    break;
+                case '}':
+                    depth--;
+                    if (depth == 0)
+                        return source[start..(index + 1)];
+                    break;
+            }
+        }
+
+        throw new InvalidDataException($"Method '{signature}' has no balanced closing brace.");
+    }
+
+    private static string FindRepoFile(string relativePath)
+        => Path.Combine(FindRepoRoot(), relativePath);
+
+    private static string FindRepoRoot()
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+            {
+                return directory.FullName;
+            }
+
+            directory = directory.Parent;
+        }
+
+        throw new DirectoryNotFoundException(
+            $"Could not locate repository root from '{AppContext.BaseDirectory}'.");
+    }
+}
diff --git a/tests/ARSAS.Tests/ProductionFatM7CleanupRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatM7CleanupRegressionTests.cs
new file mode 100644
index 000000000..f0e993000
--- /dev/null
+++ b/tests/ARSAS.Tests/ProductionFatM7CleanupRegressionTests.cs
@@ -0,0 +1,103 @@
+namespace ARSAS.Tests;
+
+public sealed class ProductionFatM7CleanupRegressionTests
+{
+    [Fact]
+    public void ObsoleteNativeFatRuntime_IsNotASecondProductionAuthority()
+    {
+        var repoRoot = FindRepoRoot();
+        var bridge = File.ReadAllText(Path.Combine(repoRoot, "MainWindow.NativeFatWorkspace.cs"));
+        var productionTab = File.ReadAllText(Path.Combine(repoRoot, "MainWindow.ProductionFatTab.cs"));
+
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.NativeFatExport.cs")),
+            "The retired native FAT export must not return; production FAT owns report/export delivery.");
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.NativeFatReportPreview.cs")),
+            "The retired native FAT side-panel preview must not return. Production FAT owns one in-place report preview.");
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.NativeFatHistoryInspector.cs")),
+            "The retired side-panel history inspector must not return as a second FAT presentation stack.");
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.NativeFatExplorerSync.cs")),
+            "The retired native FAT Explorer reconciler must not return; Engineering selection and production FAT own synchronization.");
+        Assert.False(
+            File.Exists(Path.Combine(repoRoot, "MainWindow.NativeFatPersistenceSafety.cs")),
+            "The retired native FAT persistence runtime must not return; production FAT storage remains the persistence authority.");
+
+        // MainWindow.NativeFatWorkspace.cs is now only a compatibility bridge for the
+        // canonical seventh shell slot. It must never regain its own FAT runtime/state.
+        Assert.Contains("private const int NativeFatWorkspaceIndex = 6", bridge, StringComparison.Ordinal);
+        Assert.Contains("QueueNativeFatNavigationGeometry", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("DataGrid", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("DispatcherTimer", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("NativeFatStateStore", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("NativeFatSignalRow", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("NativeFatCapture", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("SaveNativeFatStateAsync", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("BuildNativeFatWorkspaceContent", bridge, StringComparison.Ordinal);
+        Assert.DoesNotContain("RegisterNativeFatWorkspace", bridge, StringComparison.Ordinal);
+
+        // Production FAT mounts directly into the canonical XAML slot. There must be no
+        // compatibility-field handshake with the retired native runtime.
+        Assert.Contains("NativeFatTab.Content = BuildProductionFatPermanentHost();", productionTab, StringComparison.Ordinal);
+        Assert.Contains("NativeFatTab.Content = surface;", productionTab, StringComparison.Ordinal);
+        Assert.Contains("NavNativeFatButton.ToolTip", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatInstalled", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatTab", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("_nativeFatNavButton", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("AttachNativeFatObservedDevice", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("NativeFat_MainWindowPropertyChanged", productionTab, StringComparison.Ordinal);
+        Assert.DoesNotContain("NativeFat_MainTabsSelectionChanged", productionTab, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void ProductionReportPreviewAndExport_RemainSingleAuthority()
+    {
+        var repoRoot = FindRepoRoot();
+        var productionPreview = File.ReadAllText(Path.Combine(repoRoot, "IoListTestingWindow.PrintPreview.cs"));
+        var embeddedHost = File.ReadAllText(Path.Combine(repoRoot, "IoListTestingWindow.EmbeddedEngineeringHost.cs"));
+        var pdfService = File.ReadAllText(Path.Combine(repoRoot, "Services", "IoTesting", "IoFatPdfReportService.cs"));
+
+        Assert.Contains("BuildPrintPreviewHost", productionPreview, StringComparison.Ordinal);
+        Assert.Contains("IoFatReportPreviewDocumentBuilder.Build(scopedProject, draft)", productionPreview, StringComparison.Ordinal);
+        Assert.Contains("InstallPerIedPrintPreview();", embeddedHost, StringComparison.Ordinal);
+        Assert.Contains("_printPreviewToggle = WorkspacePreviewToggle;", embeddedHost, StringComparison.Ordinal);
+        Assert.Contains("BuildLayout", pdfService, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void Cleanup_DoesNotRetireProductionCapturePreflightOrEngineeringAcquisitionAuthority()
+    {
+        var bootstrap = File.ReadAllText(FindRepoFile("MainWindow.ProductionFatEngineeringBootstrap.cs"));
+        var adapter = File.ReadAllText(FindRepoFile("Services/IoTesting/IoFatProductionControllerAdapter.cs"));
+        var contract = File.ReadAllText(FindRepoFile("docs/FAT_ENGINEERING_WORKSTATION_CONTRACT.md"));
+
+        Assert.Contains("AdoptEngineeringRuntimeWorkspaces", bootstrap, StringComparison.Ordinal);
+        Assert.Contains("IoTestSessionPreflight.Validate", adapter, StringComparison.Ordinal);
+        Assert.Contains("IoFatProductionControllerAdapter", adapter, StringComparison.Ordinal);
+        Assert.Contains("production FAT", contract, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("Engineering", contract, StringComparison.OrdinalIgnoreCase);
+    }
+
+    private static string FindRepoFile(string relativePath)
+        => Path.Combine(FindRepoRoot(), relativePath);
+
+    private static string FindRepoRoot()
+    {
+        DirectoryInfo? directory = new(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "MainWindow.xaml")) &&
+                Directory.Exists(Path.Combine(directory.FullName, "tests", "ARSAS.Tests")))
+            {
+                return directory.FullName;
+            }
+
+            directory = directory.Parent;
+        }
+
+        throw new DirectoryNotFoundException(
+            $"Could not locate repository root from '{AppContext.BaseDirectory}'.");
+    }
+}
diff --git a/tests/ARSAS.Tests/ProductionFatP0FieldRegressionTests.cs b/tests/ARSAS.Tests/ProductionFatP0FieldRegressionTests.cs
index 2a220125e..4368dc587 100644
--- a/tests/ARSAS.Tests/ProductionFatP0FieldRegressionTests.cs
+++ b/tests/ARSAS.Tests/ProductionFatP0FieldRegressionTests.cs
@@ -69,6 +69,28 @@ public void EngineeringBootstrap_AppliesStaticCleanupBeforeProductionWindowIsSho
         Assert.True(show > retire, "Static DataSet cleanup must complete before the production FAT workspace/session can be exposed.");
     }
 
+    [Fact]
+    public void EngineeringBootstrap_ReusesDescribedSourcesWhileStagingStillVerifiesSha256()
+    {
+        var projection = Read("Services/IoTesting/IoFatEngineeringWorkspaceProjectionService.cs");
+        var bootstrap = Read("MainWindow.ProductionFatEngineeringBootstrap.cs");
+        var bootstrapService = Read("Services/IoTesting/IoTestWorkspaceBootstrapService.cs");
+        var persistence = Read("Services/IoTesting/IoTestProjectPersistenceService.cs");
+        var sourceWorkspace = Read("Services/IoTesting/IoFatSourceWorkspaceService.cs");
+
+        Assert.Contains("IReadOnlyList<IoFatDescribedSource> DescribedSources", projection, StringComparison.Ordinal);
+        Assert.Equal(
+            1,
+            projection.Split("IoFatSourceWorkspaceService.DescribeAsync", StringSplitOptions.None).Length - 1);
+        Assert.Contains("projection.DescribedSources", bootstrap, StringComparison.Ordinal);
+        Assert.Contains("IoTestWorkspaceBootstrapService.OpenDescribedSourcesAsync", bootstrap, StringComparison.Ordinal);
+        Assert.Contains("OpenDescribedSourcesAsync", bootstrapService, StringComparison.Ordinal);
+        Assert.Contains("IoTestWorkspacePersistence.OpenDescribedSourcesAsync", bootstrapService, StringComparison.Ordinal);
+        Assert.Contains("StageDescribedAsync", persistence, StringComparison.Ordinal);
+        Assert.Contains("CopyVerifiedAsync", sourceWorkspace, StringComparison.Ordinal);
+        Assert.Contains("VerifyHash(bytes, expectedSha256", sourceWorkspace, StringComparison.Ordinal);
+    }
+
     [Fact]
     public void EmbeddedAutomaticBootstrap_NeverHidesEngineeringWindow()
     {
@@ -78,7 +100,7 @@ public void EmbeddedAutomaticBootstrap_NeverHidesEngineeringWindow()
         Assert.Contains("ShouldKeepEngineeringVisibleDuringProductionFatBootstrap", source, StringComparison.Ordinal);
         Assert.Contains("_productionFatEngineeringBootstrapBusy", source, StringComparison.Ordinal);
         Assert.Contains("ProductionFatTabReady", source, StringComparison.Ordinal);
-        Assert.Contains("MainTabs.SelectedIndex == NativeFatWorkspaceIndex", source, StringComparison.Ordinal);
+        Assert.DoesNotContain("MainTabs.SelectedIndex == NativeFatWorkspaceIndex", source, StringComparison.Ordinal);
         Assert.Contains("base.Hide();", source, StringComparison.Ordinal);
     }