feat(terminal): add SGR double underline (attribute 21)

CHANGELOG.md

@@ -6,6 +6,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ## [Unreleased]
 
 ### Added
+- SGR double underline (attribute 21), cleared by SGR 24 along with single underline
 - add `--maximized` and `--fullscreen` flags to start the window in that mode
 
 ### Changed

src/renderer/draw_fns.rs

@@ -20,6 +20,7 @@ pub(super) static BLANK_CELL: Cell = Cell {
     overline: false,
     reverse: false,
     blink: false,
+    double_underline: false,
     wide: false,
     wide_cont: false,
     url: None,

src/renderer/text.rs

@@ -1018,27 +1018,18 @@ fn draw_cell_decorations(
         dim_color(color_u32(fg), dim_factor)
     };
 
+    let hline =
+        |buf: &mut [u32], y: u32| draw_clipped_hline(buf, buf_width, y, cell_x, draw_w, clip, fg32);
     if cell.underline {
-        draw_clipped_hline(
-            buf,
-            buf_width,
-            cell_y + m.cell_height.saturating_sub(2),
-            cell_x,
-            draw_w,
-            clip,
-            fg32,
-        );
+        hline(buf, cell_y + m.cell_height.saturating_sub(2));
+    }
+    if cell.double_underline {
+        let base = cell_y + m.cell_height.saturating_sub(1);
+        hline(buf, base);
+        hline(buf, base.saturating_sub(2));
     }
     if cell.strikethrough {
-        draw_clipped_hline(
-            buf,
-            buf_width,
-            cell_y + m.cell_height / 2,
-            cell_x,
-            draw_w,
-            clip,
-            fg32,
-        );
+        hline(buf, cell_y + m.cell_height / 2);
     }
     if cell.overline {
         draw_clipped_hline(buf, buf_width, cell_y, cell_x, draw_w, clip, fg32);

src/renderer/text_test.rs

@@ -756,6 +756,18 @@ fn draw_pane_with_underline_cell_does_not_panic() {
     do_draw(&mut r, &[pane], &InputMode::Insert);
 }
 
+#[test]
+fn draw_pane_with_double_underline_cell_does_not_panic() {
+    let mut r = make_renderer();
+    let m = r.make_metrics(Physical(16.0));
+    let (cols, rows) = m.grid_size_for(800, 600u32.saturating_sub(44));
+    let mut grid = make_grid(cols, rows);
+    grid.write_char('D');
+    grid.cell_mut(0, 0).double_underline = true;
+    let pane = make_pane(&grid, &m);
+    do_draw(&mut r, &[pane], &InputMode::Insert);
+}
+
 #[test]
 fn draw_pane_osc8_link_without_hover_does_not_panic() {
     let mut r = make_renderer();

src/terminal/grid.rs

@@ -66,6 +66,7 @@ pub struct Cell {
     pub overline: bool,
     pub reverse: bool,
     pub blink: bool,
+    pub double_underline: bool,
     /// True when this is the left half of a 2-column wide character.
     pub wide: bool,
     /// True when this is the right (placeholder) half of a wide character.
@@ -88,6 +89,7 @@ impl Default for Cell {
             overline: false,
             reverse: false,
             blink: false,
+            double_underline: false,
             wide: false,
             wide_cont: false,
             url: None,
@@ -108,6 +110,7 @@ fn cell_with_colors(c: char, fg: Color, bg: Color) -> Cell {
         overline: false,
         reverse: false,
         blink: false,
+        double_underline: false,
         wide: false,
         wide_cont: false,
         url: None,
@@ -127,6 +130,7 @@ struct SavedCursor {
     overline: bool,
     reverse: bool,
     blink: bool,
+    double_underline: bool,
 }
 
 struct SavedScreen {
@@ -148,6 +152,7 @@ struct SavedScreen {
     overline: bool,
     reverse: bool,
     blink: bool,
+    double_underline: bool,
     scrollback: VecDeque<Vec<Cell>>,
     scrollback_wrapped: VecDeque<bool>,
     row_wrapped: Vec<bool>,
@@ -173,6 +178,7 @@ pub struct Grid {
     pub overline: bool,
     pub reverse: bool,
     pub blink: bool,
+    pub double_underline: bool,
     // DECSC/DECRC saved cursor (ESC 7 / ESC 8)
     decsc: Option<SavedCursor>,
     // Scrollback: lines that have scrolled off the top (oldest first)
@@ -268,6 +274,7 @@ impl Grid {
             overline: false,
             reverse: false,
             blink: false,
+            double_underline: false,
             decsc: None,
             scrollback: VecDeque::new(),
             scrollback_wrapped: VecDeque::new(),
@@ -326,6 +333,7 @@ impl Grid {
             overline: self.overline,
             reverse: self.reverse,
             blink: self.blink,
+            double_underline: self.double_underline,
             scrollback: std::mem::take(&mut self.scrollback),
             scrollback_wrapped: std::mem::take(&mut self.scrollback_wrapped),
             row_wrapped: std::mem::replace(&mut self.row_wrapped, vec![false; self.rows]),
@@ -375,6 +383,7 @@ impl Grid {
             self.overline = saved.overline;
             self.reverse = saved.reverse;
             self.blink = saved.blink;
+            self.double_underline = saved.double_underline;
             self.scrollback = saved.scrollback;
             self.scrollback_wrapped = saved.scrollback_wrapped;
             self.row_wrapped = saved.row_wrapped;
@@ -638,6 +647,7 @@ impl Grid {
             overline: self.overline,
             reverse: self.reverse,
             blink: self.blink,
+            double_underline: self.double_underline,
             wide,
             url: self.current_url.clone(),
             ..cell_with_colors(c, self.fg, self.bg)
@@ -974,6 +984,7 @@ impl Grid {
         self.overline = false;
         self.reverse = false;
         self.blink = false;
+        self.double_underline = false;
     }
 
     pub fn save_cursor(&mut self) {
@@ -990,6 +1001,7 @@ impl Grid {
             overline: self.overline,
             reverse: self.reverse,
             blink: self.blink,
+            double_underline: self.double_underline,
         });
     }
 
@@ -1007,6 +1019,7 @@ impl Grid {
             self.overline = s.overline;
             self.reverse = s.reverse;
             self.blink = s.blink;
+            self.double_underline = s.double_underline;
         }
     }
 

src/terminal/parser.rs

@@ -66,27 +66,19 @@ struct Performer<'a> {
 
 impl Performer<'_> {
     fn handle_dec_private_modes(&mut self, action: char, p0: u16) {
+        // Only DECSET (h) / DECRST (l) toggle modes; the `('h' | 'l', _)` patterns
+        // keep other actions out of these arms, so `on` is only used when relevant.
+        let on = action == 'h';
         match (action, p0) {
-            ('h', 1) => self.grid.application_cursor_keys = true,
-            ('l', 1) => self.grid.application_cursor_keys = false,
-            ('h', 7) => self.grid.autowrap = true,
-            ('l', 7) => self.grid.autowrap = false,
-            ('h', 25) => self.grid.cursor_visible = true,
-            ('l', 25) => self.grid.cursor_visible = false,
-            ('h', 1000) => self.grid.mouse_mode = 1000,
-            ('l', 1000) => self.grid.mouse_mode = 0,
-            ('h', 1002) => self.grid.mouse_mode = 1002,
-            ('l', 1002) => self.grid.mouse_mode = 0,
-            ('h', 1003) => self.grid.mouse_mode = 1003,
-            ('l', 1003) => self.grid.mouse_mode = 0,
-            ('h', 1004) => self.grid.focus_report = true,
-            ('l', 1004) => self.grid.focus_report = false,
-            ('h', 1006) => self.grid.mouse_sgr = true,
-            ('l', 1006) => self.grid.mouse_sgr = false,
+            ('h' | 'l', 1) => self.grid.application_cursor_keys = on,
+            ('h' | 'l', 7) => self.grid.autowrap = on,
+            ('h' | 'l', 25) => self.grid.cursor_visible = on,
+            ('h' | 'l', 1000 | 1002 | 1003) => self.grid.mouse_mode = if on { p0 } else { 0 },
+            ('h' | 'l', 1004) => self.grid.focus_report = on,
+            ('h' | 'l', 1006) => self.grid.mouse_sgr = on,
+            ('h' | 'l', 2004) => self.grid.bracketed_paste = on,
             ('h', 1049) => self.grid.enter_alternate_screen(),
             ('l', 1049) => self.grid.exit_alternate_screen(),
-            ('h', 2004) => self.grid.bracketed_paste = true,
-            ('l', 2004) => self.grid.bracketed_paste = false,
             _ => {}
         }
     }
@@ -178,8 +170,12 @@ impl Performer<'_> {
                     self.grid.bold = false;
                     self.grid.dim = false;
                 }
+                21 => self.grid.double_underline = true,
                 23 => self.grid.italic = false,
-                24 => self.grid.underline = false,
+                24 => {
+                    self.grid.underline = false;
+                    self.grid.double_underline = false;
+                }
                 25 => self.grid.blink = false,
                 27 => self.grid.reverse = false,
                 29 => self.grid.strikethrough = false,

src/terminal/parser_test.rs

@@ -201,6 +201,33 @@ fn sgr_reset_clears_attributes() {
     assert!(!p.grid.blink);
 }
 
+#[test]
+fn sgr_double_underline_stamped_on_cell() {
+    let mut p = make_parser(10, 5);
+    p.process(b"\x1b[21mA");
+    assert!(p.grid.cell(0, 0).double_underline);
+    assert!(p.grid.double_underline);
+}
+
+#[test]
+fn sgr_24_clears_single_and_double_underline() {
+    let mut p = make_parser(10, 5);
+    p.process(b"\x1b[4;21m"); // both underline and double underline on
+    assert!(p.grid.underline);
+    assert!(p.grid.double_underline);
+    p.process(b"\x1b[24m");
+    assert!(!p.grid.underline);
+    assert!(!p.grid.double_underline);
+}
+
+#[test]
+fn sgr_reset_clears_double_underline() {
+    let mut p = make_parser(10, 5);
+    p.process(b"\x1b[21m");
+    p.process(b"\x1b[0m");
+    assert!(!p.grid.double_underline);
+}
+
 #[test]
 fn erase_in_line_to_end() {
     let mut p = make_parser(10, 5);