respect new underline color when rendering

```
printf "\x1b[4m\x1b[58;2;255;0;0mred underline\x1b[0m"
```

prints "red underline" in the foreground color, with an
underline that is bright red `rgb(255, 0, 0)`.

refs: #415

wezterm-gui/src/gui/fragment.glsl

@@ -8,6 +8,7 @@ in vec3 o_hsv;
 in vec4 o_bg_color;
 in vec4 o_cursor_color;
 in vec4 o_fg_color;
+in vec4 o_underline_color;
 
 uniform mat4 projection;
 uniform bool window_bg_layer;
@@ -89,9 +90,9 @@ void main() {
     vec4 under_color = texture(atlas_nearest_sampler, o_underline);
     if (under_color.a != 0.0) {
         // if the underline glyph isn't transparent in this position then
-        // we take the text fg color, otherwise we'll leave the color
+        // we take the underline color, otherwise we'll leave the color
         // at the background color.
-        color = o_fg_color;
+        color = o_underline_color;
     }
 
     // Similar to the above: if the cursor texture isn't transparent

wezterm-gui/src/gui/quad.rs

@@ -30,6 +30,7 @@ pub struct Vertex {
     pub cursor_color: (f32, f32, f32, f32),
     pub bg_color: (f32, f32, f32, f32),
     pub fg_color: (f32, f32, f32, f32),
+    pub underline_color: (f32, f32, f32, f32),
     pub hsv: (f32, f32, f32),
     // We use a float for this because I can't get
     // bool or integer values to work:
@@ -55,6 +56,7 @@ pub struct Vertex {
     cursor_color,
     bg_color,
     fg_color,
+    underline_color,
     hsv,
     has_color
 );
@@ -171,6 +173,13 @@ impl<'a> Quad<'a> {
         }
     }
 
+    pub fn set_underline_color(&mut self, color: Color) {
+        let color = color.to_tuple_rgba();
+        for v in self.vert.iter_mut() {
+            v.underline_color = color;
+        }
+    }
+
     pub fn set_bg_color(&mut self, color: Color) {
         let color = color.to_tuple_rgba();
         for v in self.vert.iter_mut() {

wezterm-gui/src/gui/termwindow.rs

@@ -2434,6 +2434,7 @@ impl TermWindow {
                 };
 
                 quad.set_fg_color(foreground);
+                quad.set_underline_color(foreground);
                 quad.set_bg_color(background);
                 quad.set_hsv(None);
                 quad.set_texture(texture_rect);
@@ -2563,6 +2564,7 @@ impl TermWindow {
 
             quad.set_bg_color(color);
             quad.set_fg_color(color);
+            quad.set_underline_color(color);
             quad.set_position(left, top, right, bottom);
             quad.set_texture(white_space);
             quad.set_texture_adjust(0., 0., 0., 0.);
@@ -2594,6 +2596,7 @@ impl TermWindow {
             quad.set_hsv(config.window_background_image_hsb);
             quad.set_cursor_color(color);
             quad.set_fg_color(color);
+            quad.set_underline_color(color);
             quad.set_bg_color(color);
         }
 
@@ -2803,31 +2806,40 @@ impl TermWindow {
 
             let bg_is_default = attrs.background == ColorAttribute::Default;
             let bg_color = params.palette.resolve_bg(attrs.background);
-            let fg_color = match attrs.foreground {
-                wezterm_term::color::ColorAttribute::Default => {
-                    if let Some(fg) = style.foreground {
-                        fg
-                    } else {
-                        params.palette.resolve_fg(attrs.foreground)
+
+            fn resolve_fg_color_attr(
+                attrs: &CellAttributes,
+                fg: &ColorAttribute,
+                params: &RenderScreenLineOpenGLParams,
+                style: &config::TextStyle,
+            ) -> RgbColor {
+                match fg {
+                    wezterm_term::color::ColorAttribute::Default => {
+                        if let Some(fg) = style.foreground {
+                            fg
+                        } else {
+                            params.palette.resolve_fg(attrs.foreground)
+                        }
                     }
+                    wezterm_term::color::ColorAttribute::PaletteIndex(idx)
+                        if *idx < 8 && params.config.bold_brightens_ansi_colors =>
+                    {
+                        // For compatibility purposes, switch to a brighter version
+                        // of one of the standard ANSI colors when Bold is enabled.
+                        // This lifts black to dark grey.
+                        let idx = if attrs.intensity() == wezterm_term::Intensity::Bold {
+                            *idx + 8
+                        } else {
+                            *idx
+                        };
+                        params
+                            .palette
+                            .resolve_fg(wezterm_term::color::ColorAttribute::PaletteIndex(idx))
+                    }
+                    _ => params.palette.resolve_fg(*fg),
                 }
-                wezterm_term::color::ColorAttribute::PaletteIndex(idx)
-                    if idx < 8 && params.config.bold_brightens_ansi_colors =>
-                {
-                    // For compatibility purposes, switch to a brighter version
-                    // of one of the standard ANSI colors when Bold is enabled.
-                    // This lifts black to dark grey.
-                    let idx = if attrs.intensity() == wezterm_term::Intensity::Bold {
-                        idx + 8
-                    } else {
-                        idx
-                    };
-                    params
-                        .palette
-                        .resolve_fg(wezterm_term::color::ColorAttribute::PaletteIndex(idx))
-                }
-                _ => params.palette.resolve_fg(attrs.foreground),
-            };
+            }
+            let fg_color = resolve_fg_color_attr(&attrs, &attrs.foreground, &params, &style);
 
             let (fg_color, bg_color, bg_is_default) = {
                 let mut fg = fg_color;
@@ -2843,6 +2855,11 @@ impl TermWindow {
             };
 
             let glyph_color = rgbcolor_to_window_color(fg_color);
+            let underline_color = match attrs.underline_color() {
+                ColorAttribute::Default => fg_color,
+                c => resolve_fg_color_attr(&attrs, &c, &params, &style),
+            };
+            let underline_color = rgbcolor_to_window_color(underline_color);
 
             let bg_color = rgbcolor_alpha_to_window_color(
                 bg_color,
@@ -3015,6 +3032,7 @@ impl TermWindow {
 
                         quad.set_hsv(hsv);
                         quad.set_fg_color(glyph_color);
+                        quad.set_underline_color(underline_color);
                         quad.set_bg_color(bg_color);
                         quad.set_texture(texture_rect);
                         quad.set_texture_adjust(0., 0., 0., 0.);
@@ -3065,6 +3083,7 @@ impl TermWindow {
                     quad.set_texture(texture_rect);
                     quad.set_texture_adjust(left, top, right, bottom);
                     quad.set_underline(underline_tex_rect);
+                    quad.set_underline_color(underline_color);
                     quad.set_hsv(hsv);
                     quad.set_has_color(glyph.has_color);
                     quad.set_cursor(
@@ -3121,6 +3140,7 @@ impl TermWindow {
 
             quad.set_bg_color(bg_color);
             quad.set_fg_color(glyph_color);
+            quad.set_underline_color(glyph_color);
             quad.set_texture(white_space);
             quad.set_texture_adjust(0., 0., 0., 0.);
             quad.set_underline(white_space);

wezterm-gui/src/gui/vertex.glsl

@@ -6,6 +6,7 @@ in vec2 tex;
 in vec2 underline;
 in vec4 bg_color;
 in vec4 fg_color;
+in vec4 underline_color;
 in float has_color;
 in vec2 cursor;
 in vec4 cursor_color;
@@ -24,6 +25,7 @@ out vec3 o_hsv;
 out vec4 o_bg_color;
 out vec4 o_cursor_color;
 out vec4 o_fg_color;
+out vec4 o_underline_color;
 
 // Returns a position that is outside of the viewport,
 // such that this vertex effectively won't contribute
@@ -39,6 +41,7 @@ void main() {
     o_fg_color = fg_color;
     o_bg_color = bg_color;
     o_underline = underline;
+    o_underline_color = underline_color;
     o_cursor = cursor;
     o_cursor_color = cursor_color;
     o_hsv = hsv;