diff --git a/src/shell/RunDetail.tsx b/src/shell/RunDetail.tsx
index 88085c00..223c852d 100644
--- a/src/shell/RunDetail.tsx
+++ b/src/shell/RunDetail.tsx
@@ -15,6 +15,8 @@ import {
   BackIcon,
   ChartIcon,
   ChevronDownIcon,
+  CompressIcon,
+  ExpandIcon,
   FileIcon,
   PlayIcon,
   StopIcon,
@@ -80,6 +82,77 @@ const RunDetail = ({ runId }: { runId: string }) => {
       return next;
     });
 
+  // Fullscreen viewport: native Fullscreen API when available, an in-app
+  // fixed overlay ("immersive") when it is not (or the request is denied).
+  const viewportRef = useRef<HTMLDivElement | null>(null);
+  const [nativeFullscreen, setNativeFullscreen] = useState(false);
+  const [immersive, setImmersive] = useState(false);
+  const expanded = nativeFullscreen || immersive;
+
+  useEffect(() => {
+    const onChange = () =>
+      setNativeFullscreen(
+        document.fullscreenElement != null &&
+          document.fullscreenElement === viewportRef.current,
+      );
+    document.addEventListener("fullscreenchange", onChange);
+    return () => document.removeEventListener("fullscreenchange", onChange);
+  }, []);
+
+  useEffect(() => {
+    if (!immersive) {
+      return;
+    }
+    // Native fullscreen exits on Esc by itself; the overlay needs a handler.
+    const onKey = (event: KeyboardEvent) => {
+      if (event.key === "Escape") {
+        setImmersive(false);
+      }
+    };
+    window.addEventListener("keydown", onKey);
+    // The overlay resizes the viewport without a window resize; nudge the
+    // remaining window-resize listeners (the 3D canvas itself polls its
+    // container size every frame and doesn't need it).
+    window.dispatchEvent(new Event("resize"));
+    return () => {
+      window.removeEventListener("keydown", onKey);
+      window.dispatchEvent(new Event("resize"));
+    };
+  }, [immersive]);
+
+  // Leave fullscreen when the live run ends: native fullscreen's top layer
+  // hides everything outside the viewport (toasts included), and the
+  // completion UI lives outside it.
+  useEffect(() => {
+    if (live) {
+      return;
+    }
+    setImmersive(false);
+    if (document.fullscreenElement) {
+      void document.exitFullscreen().catch(() => {});
+    }
+  }, [live]);
+
+  const toggleFullscreen = () => {
+    const element = viewportRef.current;
+    if (!element) {
+      return;
+    }
+    if (nativeFullscreen) {
+      void document.exitFullscreen().catch(() => {});
+      return;
+    }
+    if (immersive) {
+      setImmersive(false);
+      return;
+    }
+    if (typeof element.requestFullscreen === "function") {
+      element.requestFullscreen().catch(() => setImmersive(true));
+    } else {
+      setImmersive(true);
+    }
+  };
+
   // Finished runs: persisted log + frame + output list from storage.
   useEffect(() => {
     if (live || !dirName) {
@@ -337,14 +410,25 @@ const RunDetail = ({ runId }: { runId: string }) => {
 
         {/* Viewport */}
         <div
+          ref={viewportRef}
           data-testid="run-viewport"
-          style={{
-            flex: 1,
-            position: "relative",
-            background: "var(--viewport)",
-            overflow: "hidden",
-            minHeight: 0,
-          }}
+          style={
+            immersive
+              ? {
+                  position: "fixed",
+                  inset: 0,
+                  zIndex: 1000,
+                  background: "var(--viewport)",
+                  overflow: "hidden",
+                }
+              : {
+                  flex: 1,
+                  position: "relative",
+                  background: "var(--viewport)",
+                  overflow: "hidden",
+                  minHeight: 0,
+                }
+          }
         >
           {live ? (
             <View visible pane />
@@ -470,6 +554,63 @@ const RunDetail = ({ runId }: { runId: string }) => {
               </>
             )}
           </div>
+          {/* Stop stays reachable while the run screen chrome is hidden. */}
+          {expanded && live && (
+            <button
+              onClick={() => ui.stopRun()}
+              data-testid="viewport-stop"
+              title="Stop the run"
+              aria-label="Stop the run"
+              className="shell-danger-hover"
+              style={{
+                position: "absolute",
+                top: 14,
+                right: 58,
+                height: 32,
+                display: "inline-flex",
+                alignItems: "center",
+                gap: 7,
+                padding: "0 12px",
+                border: "none",
+                borderRadius: 9,
+                background: "var(--bad)",
+                color: "#fff",
+                fontWeight: 700,
+                fontSize: 12.5,
+                cursor: "pointer",
+                fontFamily: "inherit",
+              }}
+            >
+              <StopIcon />
+              Stop
+            </button>
+          )}
+          {/* Fullscreen toggle */}
+          <button
+            onClick={toggleFullscreen}
+            data-testid="viewport-fullscreen"
+            title={expanded ? "Exit full screen (Esc)" : "Full screen"}
+            aria-label={expanded ? "Exit full screen" : "Full screen"}
+            style={{
+              position: "absolute",
+              top: 14,
+              right: 16,
+              width: 32,
+              height: 32,
+              display: "flex",
+              alignItems: "center",
+              justifyContent: "center",
+              padding: 0,
+              border: "1px solid rgba(255,255,255,0.18)",
+              borderRadius: 9,
+              background: "rgba(10,12,17,0.55)",
+              color: "rgba(255,255,255,0.85)",
+              cursor: "pointer",
+              backdropFilter: "blur(4px)",
+            }}
+          >
+            {expanded ? <CompressIcon /> : <ExpandIcon />}
+          </button>
         </div>
 
         {/* Console strip (collapsible: the header bar always stays visible,
diff --git a/src/shell/icons.tsx b/src/shell/icons.tsx
index 681c7a99..5abb495f 100644
--- a/src/shell/icons.tsx
+++ b/src/shell/icons.tsx
@@ -251,6 +251,20 @@ export const ChevronRightIcon = ({ size = 16 }: IconProps) =>
 export const BackIcon = ({ size = 14 }: IconProps) =>
   stroke(size, <path d="m15 18-6-6 6-6" />, 2);
 
+export const ExpandIcon = ({ size = 15 }: IconProps) =>
+  stroke(
+    size,
+    <path d="M15 3h6v6M9 21H3v-6M21 3l-7 7M3 21l7-7" />,
+    2,
+  );
+
+export const CompressIcon = ({ size = 15 }: IconProps) =>
+  stroke(
+    size,
+    <path d="M20 10h-6V4M4 14h6v6M14 10l7-7M10 14l-7 7" />,
+    2,
+  );
+
 export const DotsIcon = ({ size = 16 }: IconProps) => (
   <svg width={size} height={size} viewBox="0 0 24 24" fill="currentColor" aria-hidden>
     <circle cx="5" cy="12" r="1.7" />