terminal_element: Skip contrast adjustment for 24-bit color (#52162)

Nihal Kumar created 2 days ago

## Context

Red and blue have inherently low perceptual luminance in APCA — pure
default Lc 45 threshold. This caused ensure_minimum_contrast to lighten
them, washing out the color to pink/lavender.

When an application explicitly specifies an RGB value via
ESC[38;2;R;G;Bm,
that color should be rendered as-is. Named and indexed colors continue
to be contrast-adjusted since they come from theme mappings



[zed-colors-before.webm](https://github.com/user-attachments/assets/1adb8c95-5dec-4744-b6f8-38f085602ef4)


[zed-colors-after.webm](https://github.com/user-attachments/assets/1968db26-9c02-48cf-9542-86a6c39899c0)


Closes: #50396 

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## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed true-color render in terminal

Change summary

crates/terminal_view/src/terminal_element.rs | 42 ++++++++++++++++++++-
1 file changed, 40 insertions(+), 2 deletions(-)

Detailed changes

crates/terminal_view/src/terminal_element.rs

@@ -550,11 +550,13 @@ impl TerminalElement {
         minimum_contrast: f32,
     ) -> TextRun {
         let flags = indexed.cell.flags;
+        let is_true_color = matches!(fg, terminal::alacritty_terminal::vte::ansi::Color::Spec(_));
         let mut fg = convert_color(&fg, colors);
         let bg = convert_color(&bg, colors);
 
-        // Only apply contrast adjustment to non-decorative characters
-        if !Self::is_decorative_character(indexed.c) {
+        // Skip contrast adjustment for true-color (24-bit RGB) foregrounds — the
+        // application chose that exact color. Also skip for decorative characters.
+        if !is_true_color && !Self::is_decorative_character(indexed.c) {
             fg = ensure_minimum_contrast(fg, bg, minimum_contrast);
         }
 
@@ -1848,6 +1850,42 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_true_color_red_blue_not_washed_out_on_dark_bg() {
+        // Red and blue have inherently low perceptual luminance in APCA.
+        // Pure #ff0000 only achieves Lc ~35 against #1e1e1e — below the
+        // default Lc 45 threshold. ensure_minimum_contrast would lighten
+        // them, washing out the color. This is why cell_style skips the
+        // adjustment for Color::Spec (24-bit true color).
+        let dark_bg = gpui::Hsla {
+            h: 0.0,
+            s: 0.0,
+            l: 0.05,
+            a: 1.0,
+        };
+
+        for (name, r, g, b) in [
+            ("red", 225, 80, 80),
+            ("blue", 80, 80, 225),
+            ("pure red", 255, 0, 0),
+        ] {
+            let color = terminal::rgba_color(r, g, b);
+            let contrast = apca_contrast(color, dark_bg).abs();
+            assert!(
+                contrast < 45.0,
+                "{name} should have APCA < 45 on dark bg, got {contrast}",
+            );
+
+            let adjusted = ensure_minimum_contrast(color, dark_bg, 45.0);
+            assert!(
+                adjusted.l > color.l,
+                "{name} would be lightened by contrast adjustment (l: {} -> {})",
+                color.l,
+                adjusted.l,
+            );
+        }
+    }
+
     #[test]
     fn test_white_on_white_contrast_issue() {
         // This test reproduces the exact issue from the bug report