fix: drop the palette combination that looked unthemed

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## Unreleased
+
+### Fixed
+
+- **Some worktree colours barely looked coloured.** Every colour has to be dark
+  enough for white text to stay readable, and VS Code's default chrome is dark
+  too, so the two could drift close: the dullest, darkest quarter of the palette
+  sat ΔE 22 from an unthemed window. That combination is gone. The worst case is
+  now 27, and telling worktrees apart from each other is unaffected — collisions
+  among four stay at seven percent.
+
+  Existing worktrees change colour the next time `gwt add` or `gwt open` themes
+  them.
+
 ## 0.8.2
 
 Nothing to reinstall — the shell wrapper is unchanged.

src/lib/ideTheme.ts

@@ -14,8 +14,8 @@ const FORMATTING = { tabSize: 2, insertSpaces: true };
 // so worktrees collide by the birthday problem rather than by any flaw: with
 // four open, two are perceptually identical (ΔE < 5) about one time in five.
 //
-// Varying lightness and saturation as well turns one wheel into four, which
-// drops that to roughly one in twelve. Both variants of each keep white text
+// Varying lightness and saturation as well turns one wheel into three, which
+// drops that to roughly one in fourteen. Both variants of each keep white text
 // above WCAG AA — measured at 4.53:1 in the worst case, against the 4.5 floor —
 // so the readability the fixed values protected is not traded away for it.
 //
@@ -24,8 +24,16 @@ const FORMATTING = { tabSize: 2, insertSpaces: true };
 // determinism (the same branch would look different on another machine) and
 // breaks under two concurrent `gwt add` — a real scenario for anyone running
 // parallel agents. Not worth it for the remaining eight percent.
-const SATURATIONS = [0.6, 0.42] as const;
-const LIGHTNESSES = [0.3, 0.22] as const;
+// Three combinations, not the four that hue × lightness × saturation allows.
+// The darkest and dullest of them (0.42 / 0.22) sat ΔE 22 from VS Code's default
+// dark chrome — close enough that a themed window read as barely themed at all.
+// Dropping it lifts the worst case to 27 and costs nothing measurable:
+// collisions among four worktrees stay at seven percent either way.
+const COMBINATIONS = [
+  { saturation: 0.6, lightness: 0.3 },
+  { saturation: 0.6, lightness: 0.22 },
+  { saturation: 0.42, lightness: 0.3 },
+] as const;
 
 // A plain `h * 31 + c` hash keeps its low bits close for inputs that are close:
 // `feature-a` and `feature-b` differ by 1 in the hash and so landed 1° apart on
@@ -77,8 +85,7 @@ export function pickColor(branch: string): PaletteEntry {
   // Separate bits for each axis: the low bits already carry the hue, so reusing
   // them would tie lightness to hue and waste the extra dimension.
   const hue = h % 360;
-  const lightness = LIGHTNESSES[(h >>> 9) & 1];
-  const saturation = SATURATIONS[(h >>> 10) & 1];
+  const { saturation, lightness } = COMBINATIONS[(h >>> 9) % COMBINATIONS.length];
   return { bg: hslToHex(hue, saturation, lightness), fg: "#ffffff" };
 }
 

tests/theme.test.ts

@@ -131,6 +131,26 @@ describe("pickColor", () => {
     assert.ok(rate < 0.12, `${(rate * 100).toFixed(0)}% collided, expected under 12%`);
   });
 
+  // Telling worktrees apart from each other is not enough: a themed window has
+  // to look themed. Every colour is dark, because white text has to stay
+  // readable on it, and VS Code's default chrome is dark too — so the two can
+  // drift close. One combination did, and read as an unthemed window.
+  test("every colour is visibly different from the default dark chrome", () => {
+    const DEFAULT_CHROME = "#181818";
+    let worst = Infinity;
+    let worstBranch = "";
+    for (let i = 0; i < 2000; i++) {
+      const branch = `branch/${i}`;
+      const d = deltaE(pickColor(branch).bg, DEFAULT_CHROME);
+      if (d < worst) {
+        worst = d;
+        worstBranch = branch;
+      }
+    }
+    // Measured at 27 with three combinations, 22 with the four-way palette.
+    assert.ok(worst >= 25, `${worstBranch} is only ΔE ${worst.toFixed(0)} from the default`);
+  });
+
   test("the palette really uses more than one wheel", () => {
     const seen = new Set<string>();
     for (let i = 0; i < 500; i++) {