fix(tab-strip): ring the status dot in white on dark themes

The badge halo was drawn in the surface colour, which only reads as a
ring while the surface is light. On a dark theme it went near-black and
looked like a notch bitten out of the avatar instead of a badge sitting
on it. Detect a dark surface and give it the same white edge — and the
same white hole for the hollow Waiting dot — that light themes get.

src/ui/presets.rs

@@ -438,6 +438,13 @@ pub(crate) fn needs_edge(fill: u32, surface: Hsla) -> bool {
     contrast(fill, packed) < 1.25
 }
 
+/// Whether a surface is dark enough that a halo cut in its own colour stops
+/// reading as a ring and starts reading as a hole.
+pub(crate) fn surface_is_dark(surface: Hsla) -> bool {
+    let rgb = crate::terminal::palette::hsla_to_rgb(surface);
+    is_dark((rgb.r as u32) << 16 | (rgb.g as u32) << 8 | rgb.b as u32)
+}
+
 pub(crate) fn mix(a: u32, b: u32, t: f32) -> u32 {
     let (ar, ag, ab) = (a >> 16 & 0xff, a >> 8 & 0xff, a & 0xff);
     let (br, bg, bb) = (b >> 16 & 0xff, b >> 8 & 0xff, b & 0xff);

src/ui/tab_strip.rs

@@ -837,7 +837,15 @@ impl Tty7App {
         hollow: bool,
     ) -> gpui::AnyElement {
         let d = (size * 0.42).max(7.);
-        let bg = ring;
+        // The halo was the surface itself, which is only a ring while the
+        // surface is light — on a dark theme it went near-black and read as a
+        // notch bitten out of the avatar rather than a badge sitting on it.
+        // Light themes already ring the dot in white; give the dark ones the
+        // same white edge, and the hollow Waiting dot the same white hole.
+        let bg = match crate::ui::presets::surface_is_dark(ring) {
+            true => gpui::white(),
+            false => ring,
+        };
         if unread > 0 {
             let nd = (size * 0.72).max(13.0);
             let label = unread.min(9).to_string();