Add colored underline support

This commit adds support for colored underline and refines the dynamic
extra storage. The extra storage now is using `Arc` making cloning it way
faster compared to `Box` approach which scales really well when it comes
to cloning in `Term::write_at_cursor`, since cloning `Arc` is constant
time.

Fixes #4142.

3 files changed, 14 insertions, 3 deletions

diff --git a/alacritty/src/display/content.rs b/alacritty/src/display/content.rs
index d2da1e81..bb3c61a8 100644
--- a/alacritty/src/display/content.rs
+++ b/alacritty/src/display/content.rs
@@ -186,6 +186,7 @@ pub struct RenderableCell {
     pub fg: Rgb,
     pub bg: Rgb,
     pub bg_alpha: f32,
+    pub underline: Rgb,
     pub flags: Flags,
 }
 
@@ -251,14 +252,20 @@ impl RenderableCell {
         let cell_point = cell.point;
         let point = display::point_to_viewport(display_offset, cell_point).unwrap();
 
+        let flags = cell.flags;
+        let underline = cell
+            .underline_color()
+            .map_or(fg, |underline| Self::compute_fg_rgb(content, underline, flags));
+
         RenderableCell {
             zerowidth: cell.zerowidth().map(|zerowidth| zerowidth.to_vec()),
-            flags: cell.flags,
+            flags,
             character,
             bg_alpha,
             point,
             fg,
             bg,
+            underline,
         }
     }
 
diff --git a/alacritty/src/renderer/mod.rs b/alacritty/src/renderer/mod.rs
index cf9ee149..4a75416b 100644
--- a/alacritty/src/renderer/mod.rs
+++ b/alacritty/src/renderer/mod.rs
@@ -135,6 +135,7 @@ impl Renderer {
             bg_alpha: 1.0,
             fg,
             bg,
+            underline: fg,
         });
 
         self.draw_cells(size_info, glyph_cache, cells);
diff --git a/alacritty/src/renderer/rects.rs b/alacritty/src/renderer/rects.rs
index a4a6bf5d..27e306be 100644
--- a/alacritty/src/renderer/rects.rs
+++ b/alacritty/src/renderer/rects.rs
@@ -193,6 +193,9 @@ impl RenderLines {
             return;
         }
 
+        // The underline color escape does not apply to strikeout.
+        let color = if flag.contains(Flags::STRIKEOUT) { cell.fg } else { cell.underline };
+
         // Include wide char spacer if the current cell is a wide char.
         let mut end = cell.point;
         if cell.flags.contains(Flags::WIDE_CHAR) {
@@ -201,7 +204,7 @@ impl RenderLines {
 
         // Check if there's an active line.
         if let Some(line) = self.inner.get_mut(&flag).and_then(|lines| lines.last_mut()) {
-            if cell.fg == line.color
+            if color == line.color
                 && cell.point.column == line.end.column + 1
                 && cell.point.line == line.end.line
             {
@@ -212,7 +215,7 @@ impl RenderLines {
         }
 
         // Start new line if there currently is none.
-        let line = RenderLine { start: cell.point, end, color: cell.fg };
+        let line = RenderLine { start: cell.point, end, color };
         match self.inner.get_mut(&flag) {
             Some(lines) => lines.push(line),
             None => {