feat(terminal): an OSC 8 link rests under a faint dotted underline (#1078)

* feat(terminal): an OSC 8 link rests under a faint dotted underline

A hyperlink a program emits (OSC 8, which panes ask for via
FORCE_HYPERLINK) looked like plain text until the pointer found it.
It now carries iTerm2's faint dotted underline, drawn by the SGR 4:4
dotted painter, and swaps to the hovered link's solid line under the
pointer. A cell the program underlined or gave an underline colour
keeps its own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(terminal): an OSC 8 link's dots stay faint under the block cursor

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: l0ng-ai <24760907+l0ng-ai@users.noreply.github.com>

src/terminal/element.rs

@@ -19,6 +19,8 @@ use gpui_component::ActiveTheme as _;
 use super::view::{TerminalView, should_show_context_menu};
 use crate::core::config::Config;
 
+mod osc8_underline;
+
 const DIM_OPACITY: f32 = 0.66;
 
 /// How much of the text's own colour a link's underline keeps before the
@@ -54,6 +56,7 @@ pub(super) struct RenderCell {
     match_hit: bool,
     match_current: bool,
     link_hover: bool,
+    osc8_dots: bool,
 }
 
 impl Default for RenderCell {
@@ -74,6 +77,7 @@ impl Default for RenderCell {
             match_hit: false,
             match_current: false,
             link_hover: false,
+            osc8_dots: false,
         }
     }
 }
@@ -252,6 +256,7 @@ fn snapshot_cell(
     if flags.contains(Flags::DIM) {
         rc.fg = dim_fg(rc.fg, bgc, colors.legible_dim);
     }
+    osc8_underline::mark(&mut rc, cell);
     rc
 }
 
@@ -1708,8 +1713,10 @@ fn invert_cursor_cell(
     cell.bg = colors.caret;
     cell.draw_bg = true;
     cell.fg = ink;
+    // Keep the underline's own alpha: an OSC 8 link's held-back dots stay
+    // faint under the caret rather than turning into a full-strength line.
     if let Some(under) = cell.underline_color.as_mut() {
-        *under = ink;
+        *under = ink.opacity(under.a);
     }
     // A wide character's trailing spacer is absorbed into the lead cell's
     // background run by `paint_backgrounds`, so the fill already covers both
@@ -2004,6 +2011,7 @@ impl TerminalElement {
             let row = grid_row as usize * cols;
             for col in start.column.0..=end.column.0.min(cols.saturating_sub(1)) {
                 let cell = &mut buf[row + col];
+                osc8_underline::unmark(cell);
                 cell.link_hover = true;
                 cell.underline_color = link_underline_color(cell, link.armed);
             }
@@ -3882,7 +3890,7 @@ mod tests {
         assert_eq!(a.as_ref(), "界");
     }
 
-    fn test_colors() -> PaintColors {
+    pub(super) fn test_colors() -> PaintColors {
         let fg = Rgb {
             r: 10,
             g: 10,

src/terminal/element/osc8_underline.rs

@@ -0,0 +1,125 @@
+//! An OSC 8 link's resting decoration: iTerm2's faint dotted underline, so a
+//! `#1234` that is a link reads as one before the pointer finds it. It rides
+//! the SGR 4:4 dotted painter, so it costs what an app's own dotted run does.
+
+use alacritty_terminal::term::cell::Cell;
+
+use super::{HELD_BACK_LINK_ALPHA, RenderCell, UnderlineKind};
+
+/// A cell the application underlined, or gave an underline colour, keeps
+/// its own decoration.
+pub(super) fn mark(rc: &mut RenderCell, cell: &Cell) {
+    if rc.underline == UnderlineKind::None
+        && rc.underline_color.is_none()
+        && cell.hyperlink().is_some()
+    {
+        rc.underline = UnderlineKind::Dotted;
+        rc.underline_color = Some(rc.fg.opacity(HELD_BACK_LINK_ALPHA));
+        rc.osc8_dots = true;
+    }
+}
+
+/// Under the pointer the dots give way to the hovered link's solid line.
+pub(super) fn unmark(rc: &mut RenderCell) {
+    if std::mem::take(&mut rc.osc8_dots) {
+        rc.underline = UnderlineKind::None;
+        rc.underline_color = None;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use alacritty_terminal::index::{Column, Line, Point};
+    use alacritty_terminal::term::cell::{Flags, Hyperlink};
+
+    use alacritty_terminal::vte::ansi::Rgb;
+
+    use super::super::{
+        GlyphStyle, PaintColors, invert_cursor_cell, snapshot_cell, tests::test_colors,
+    };
+    use super::*;
+
+    fn linked(flags: Flags) -> Cell {
+        let mut cell = Cell {
+            c: '#',
+            flags,
+            ..Cell::default()
+        };
+        cell.set_hyperlink(Some(Hyperlink::new(
+            None::<&str>,
+            "https://example.com/3328".to_string(),
+        )));
+        cell
+    }
+
+    fn paint_fixture() -> ([Rgb; 256], PaintColors) {
+        ([Rgb { r: 0, g: 0, b: 0 }; 256], test_colors())
+    }
+
+    #[test]
+    fn a_hyperlink_rests_under_faint_dots() {
+        let (palette, colors) = paint_fixture();
+        let rc = snapshot_cell(
+            &linked(Flags::empty()),
+            Point::new(Line(0), Column(0)),
+            &palette,
+            &colors,
+            None,
+        );
+        assert_eq!(rc.underline, UnderlineKind::Dotted);
+        assert_eq!(
+            rc.underline_color,
+            Some(rc.fg.opacity(HELD_BACK_LINK_ALPHA))
+        );
+    }
+
+    #[test]
+    fn an_sgr_underlined_link_keeps_its_own_line_only() {
+        let (palette, colors) = paint_fixture();
+        let rc = snapshot_cell(
+            &linked(Flags::UNDERLINE),
+            Point::new(Line(0), Column(0)),
+            &palette,
+            &colors,
+            None,
+        );
+        assert_eq!(rc.underline, UnderlineKind::Single);
+        assert_eq!(rc.underline_color, None);
+        assert!(!rc.osc8_dots);
+    }
+
+    #[test]
+    fn hovering_swaps_the_dots_for_a_solid_line() {
+        let (palette, colors) = paint_fixture();
+        let mut rc = snapshot_cell(
+            &linked(Flags::empty()),
+            Point::new(Line(0), Column(0)),
+            &palette,
+            &colors,
+            None,
+        );
+        unmark(&mut rc);
+        rc.link_hover = true;
+        assert_eq!(rc.underline, UnderlineKind::None);
+        let solid = GlyphStyle::of(&rc)
+            .underline_style()
+            .expect("a solid underline");
+        assert!(!solid.wavy);
+    }
+
+    #[test]
+    fn the_dots_stay_faint_under_a_block_cursor() {
+        let (palette, colors) = paint_fixture();
+        let rc = snapshot_cell(
+            &linked(Flags::empty()),
+            Point::new(Line(0), Column(0)),
+            &palette,
+            &colors,
+            None,
+        );
+        let mut buf = vec![rc];
+        invert_cursor_cell(&mut buf, 1, 0, 0, &colors);
+        let under = buf[0].underline_color.expect("the dots keep a colour");
+        assert_eq!(under.a, buf[0].fg.a * HELD_BACK_LINK_ALPHA);
+    }
+}