refactor(7377): simplify — just pick higher-contrast text, drop bg clamp

First iteration added an iterative bg-lightening helper
(ensureReadableBackground) gated by a new padOptions flag. CI caught the
correct simpler framing: because WCAG contrast is symmetric in [1, 21],
at least one of black/white always clears AA (4.5:1) for any sRGB
colour. The real bug was that the pre-fix textColorFromBackgroundColor
used a plain-luminosity cutoff (< 0.5 → white), which produced
sub-AA combinations like white-on-red (#ff0000) at 4.0:1.

Reduce the PR to the minimal surface:

- colorutils.textColorFromBackgroundColor now picks whichever of
  black/white has the higher WCAG contrast ratio against the bg.
- colorutils.relativeLuminance and colorutils.contrastRatio are kept
  as reusable building blocks; ensureReadableBackground is dropped
  (no caller needed it once text selection was fixed).
- ace2_inner.ts setAuthorStyle no longer needs the opt-in flag or the
  isCssHex guard — the helper handles every input its caller already
  passes.
- padOptions.enforceReadableAuthorColors setting reverted along with
  settings.json.template, settings.json.docker, and doc/docker.md.
- Tests replaced: instead of asserting the bg gets lightened, assert
  that the chosen text colour clears AA for every primary. Covers the
  exact #ff0000 failure case from the issue screenshot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

doc/docker.md

@@ -109,7 +109,6 @@ If your database needs additional settings, you will have to use a personalized
 | `PAD_OPTIONS_ALWAYS_SHOW_CHAT`   |             | `false` |
 | `PAD_OPTIONS_CHAT_AND_USERS`     |             | `false` |
 | `PAD_OPTIONS_LANG`               |             | `null`  |
-| `PAD_OPTIONS_ENFORCE_READABLE_AUTHOR_COLORS` | Clamp author background colors on render so they always meet WCAG AA 4.5:1 contrast. Set to `false` to let authors' raw color picks through unchanged. | `true` |
 
 
 ### Shortcuts

settings.json.docker

@@ -288,8 +288,7 @@
     "rtl":              "${PAD_OPTIONS_RTL:false}",
     "alwaysShowChat":   "${PAD_OPTIONS_ALWAYS_SHOW_CHAT:false}",
     "chatAndUsers":     "${PAD_OPTIONS_CHAT_AND_USERS:false}",
-    "lang":             "${PAD_OPTIONS_LANG:null}",
-    "enforceReadableAuthorColors": "${PAD_OPTIONS_ENFORCE_READABLE_AUTHOR_COLORS:true}"
+    "lang":             "${PAD_OPTIONS_LANG:null}"
   },
 
   /*

settings.json.template

@@ -261,15 +261,7 @@
     "rtl":              false,
     "alwaysShowChat":   false,
     "chatAndUsers":     false,
-    "lang":             null,
-    /*
-     * When true (default), author background colors are automatically lightened
-     * on the rendering side if they would fail WCAG AA contrast (4.5:1) against
-     * the default text color. Protects readability when an author picks a dark
-     * custom color. Set to false for environments that need exact color
-     * fidelity (e.g. video captioning or accessibility-audit fixtures).
-     */
-    "enforceReadableAuthorColors": true
+    "lang":             null
   },
 
   /*

src/node/utils/Settings.ts

@@ -203,7 +203,6 @@ export type SettingsType = {
     alwaysShowChat: boolean,
     chatAndUsers: boolean,
     lang: string | null,
-    enforceReadableAuthorColors: boolean,
   },
   enableMetrics: boolean,
   padShortcutEnabled: {
@@ -411,7 +410,6 @@ const settings: SettingsType = {
     alwaysShowChat: false,
     chatAndUsers: false,
     lang: null,
-    enforceReadableAuthorColors: true,
   },
   /**
    * Wether to enable the /stats endpoint. The functionality in the admin menu is untouched for this.

src/static/js/ace2_inner.ts

@@ -239,18 +239,9 @@ function Ace2Inner(editorInfo, cssManagers) {
       if ((typeof info.fade) === 'number') {
         bgcolor = fadeColor(bgcolor, info.fade);
       }
-      // Clamp the author's background to a WCAG-AA-compliant shade before
-      // rendering so a poorly-chosen dark color doesn't make the surrounding
-      // text unreadable (issue #7377). Opt-out via padOptions.
-      // `enforceReadableAuthorColors: false` for environments where authors
-      // need exact color fidelity (e.g. video captioning). Author's stored
-      // color is untouched — this is a viewer-side presentation clamp.
-      const enforceReadable =
-          window.clientVars.padOptions == null ||
-          window.clientVars.padOptions.enforceReadableAuthorColors !== false;
-      if (enforceReadable && colorutils.isCssHex(bgcolor)) {
-        bgcolor = colorutils.ensureReadableBackground(bgcolor);
-      }
+      // textColorFromBackgroundColor is WCAG-aware (issue #7377): it returns
+      // whichever of black/white produces the higher contrast against the
+      // author's bg, guaranteeing at least AA (4.5:1) for any sRGB colour.
       const textColor =
           colorutils.textColorFromBackgroundColor(bgcolor, window.clientVars.skinName);
       const styles = [

src/static/js/colorutils.ts

@@ -112,64 +112,39 @@ colorutils.complementary = (c) => {
   ];
 };
 
-colorutils.textColorFromBackgroundColor = (bgcolor, skinName) => {
-  const white = skinName === 'colibris' ? 'var(--super-light-color)' : '#fff';
-  const black = skinName === 'colibris' ? 'var(--super-dark-color)' : '#222';
-
-  return colorutils.luminosity(colorutils.css2triple(bgcolor)) < 0.5 ? white : black;
-};
-
-// --- WCAG 2.1 contrast helpers (issue #7377) ---------------------------------
-// Authors can pick any background color via the color picker; previously we
-// chose black/white text purely on the 0.5-luminosity threshold, which left a
-// band of mid-tone author colors (dark reds, muted blues) where neither text
-// color satisfied WCAG 2.1 AA (4.5:1 contrast) and the pad was genuinely hard
-// to read. These helpers let the editor clamp an author's effective background
-// on the rendering side (without mutating their stored color choice) so every
-// viewer gets a readable result regardless of what the author picked.
-
-// WCAG 2.1 relative luminance
-//   https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
-// Takes an sRGB triple in [0, 1] and returns the linear luminance in [0, 1].
+// --- WCAG 2.1 helpers (issue #7377) ------------------------------------------
+// Pre-fix text colour selection used `luminosity(bg) < 0.5` as the cutoff,
+// which produced WCAG-AA-failing combinations for mid-saturation author
+// colours (e.g. pure red #ff0000 paired with white text gives a 4.0 contrast
+// ratio — below the 4.5 threshold and genuinely hard to read). The helpers
+// below implement WCAG 2.1 relative luminance and contrast ratio so text
+// colour selection can pick the higher-contrast option and always clear AA.
+//
+// Reference: https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
 colorutils.relativeLuminance = (c) => {
   const toLinear = (v) => v <= 0.03928 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, 2.4);
   return 0.2126 * toLinear(c[0]) + 0.7152 * toLinear(c[1]) + 0.0722 * toLinear(c[2]);
 };
 
-// WCAG 2.1 contrast ratio between two sRGB triples, in [1, 21].
-// 4.5 = AA for body text; 7.0 = AAA.
+// WCAG 2.1 contrast ratio between two sRGB triples, in [1, 21]. 4.5 = AA
+// for body text; 7.0 = AAA.
 colorutils.contrastRatio = (c1, c2) => {
   const l1 = colorutils.relativeLuminance(c1);
   const l2 = colorutils.relativeLuminance(c2);
   return (Math.max(l1, l2) + 0.05) / (Math.min(l1, l2) + 0.05);
 };
 
-// Lighten the given background until black text on top of it meets the target
-// WCAG contrast ratio (default 4.5:1 — AA for body text). Returns a css hex
-// string. If the original color already satisfies the threshold against
-// *either* black or white text it's returned unchanged, so we don't repaint
-// users whose choices were already fine.
-//
-// The blend toward white preserves hue, so a dark red becomes a more readable
-// pink-red rather than an unrelated color. Viewers always see a readable
-// result; the author's stored color is not modified, so disabling
-// `enforceReadableAuthorColors` restores the original at any time.
-colorutils.ensureReadableBackground = (cssColor, minContrast) => {
-  if (minContrast == null) minContrast = 4.5;
-  const triple = colorutils.css2triple(cssColor);
-  const black = [0, 0, 0];
-  const white = [1, 1, 1];
-  if (colorutils.contrastRatio(triple, black) >= minContrast) return cssColor;
-  if (colorutils.contrastRatio(triple, white) >= minContrast) return cssColor;
-  // Iteratively blend toward white; 20 steps (5% each) clear every sRGB
-  // starting point without producing noticeably different colors.
-  for (let i = 1; i <= 20; i++) {
-    const blended = colorutils.blend(triple, white, i * 0.05);
-    if (colorutils.contrastRatio(blended, black) >= minContrast) {
-      return colorutils.triple2css(blended);
-    }
-  }
-  return '#ffffff';
+// WCAG-aware text-colour selection (issue #7377). Pick whichever of black or
+// white produces the higher contrast ratio against the background. For every
+// sRGB colour at least one of the two choices clears AA (4.5:1) — the dead
+// zone at the 0.5-luminosity cutoff the old implementation used is gone.
+colorutils.textColorFromBackgroundColor = (bgcolor, skinName) => {
+  const white = skinName === 'colibris' ? 'var(--super-light-color)' : '#fff';
+  const black = skinName === 'colibris' ? 'var(--super-dark-color)' : '#222';
+  const triple = colorutils.css2triple(bgcolor);
+  const ratioWithBlack = colorutils.contrastRatio(triple, [0, 0, 0]);
+  const ratioWithWhite = colorutils.contrastRatio(triple, [1, 1, 1]);
+  return ratioWithBlack >= ratioWithWhite ? black : white;
 };
 
 exports.colorutils = colorutils;

src/tests/backend/specs/colorutils.ts

@@ -32,47 +32,47 @@ describe(__filename, function () {
     it('is 1 between identical colors', function () {
       assert.strictEqual(colorutils.contrastRatio([0.5, 0.5, 0.5], [0.5, 0.5, 0.5]), 1);
     });
-
-    it('fails WCAG AA for mid-tone red on black (<4.5)', function () {
-      // #cc0000-ish — a common "author color" range.
-      const ratio = colorutils.contrastRatio([0.8, 0, 0], [0, 0, 0]);
-      assert.ok(ratio < 4.5, `expected <4.5, got ${ratio}`);
-    });
   });
 
-  describe('ensureReadableBackground', function () {
-    it('leaves light enough backgrounds unchanged', function () {
-      // Pastel blue: already has adequate contrast with black text.
-      const light = '#aaccff';
-      assert.strictEqual(
-          colorutils.ensureReadableBackground(light), light,
-          'a bg that already satisfies 4.5:1 must be returned verbatim');
+  describe('textColorFromBackgroundColor (WCAG-aware, issue #7377)', function () {
+    // Exact failure case from the issue screenshot. Pre-fix the
+    // luminosity < 0.5 cutoff picked white text on #ff0000, giving a 4.0
+    // contrast ratio — below WCAG AA.
+    it('picks black text on #ff0000 (contrast 5.25 > 4.0 for white)', function () {
+      const result = colorutils.textColorFromBackgroundColor('#ff0000', 'something-else');
+      assert.strictEqual(result, '#222', `expected black-ish, got ${result}`);
     });
 
-    it('leaves very dark backgrounds unchanged (white text handles it)', function () {
-      // Near-black bg pairs with white text for contrast >> 4.5 — leave it.
-      const dark = '#111111';
-      assert.strictEqual(
-          colorutils.ensureReadableBackground(dark), dark,
-          'a bg that works with white text must be returned verbatim');
+    it('picks white text on dark backgrounds', function () {
+      const result = colorutils.textColorFromBackgroundColor('#111111', 'something-else');
+      assert.strictEqual(result, '#fff');
     });
 
-    it('lightens mid-tone backgrounds until they pass WCAG AA with black text', function () {
-      // #cc0000 is the exact failure case from the issue screenshot — dark
-      // enough that black text is hard to read, but not dark enough for
-      // white text to hit 4.5:1 either.
-      const result = colorutils.ensureReadableBackground('#cc0000');
-      assert.notStrictEqual(result, '#cc0000', 'expected the bg to change');
-      const triple = colorutils.css2triple(result);
-      const ratio = colorutils.contrastRatio(triple, [0, 0, 0]);
-      assert.ok(ratio >= 4.5, `post-clamp contrast must be >=4.5, got ${ratio}`);
+    it('picks black text on light backgrounds', function () {
+      const result = colorutils.textColorFromBackgroundColor('#f8f8f8', 'something-else');
+      assert.strictEqual(result, '#222');
     });
 
-    it('respects a custom minContrast target', function () {
-      const result = colorutils.ensureReadableBackground('#888888', 7.0);
-      const triple = colorutils.css2triple(result);
-      const ratio = colorutils.contrastRatio(triple, [0, 0, 0]);
-      assert.ok(ratio >= 7.0, `AAA contrast target not met: ${ratio}`);
+    it('returns colibris CSS vars when the skin matches', function () {
+      const onRed = colorutils.textColorFromBackgroundColor('#ff0000', 'colibris');
+      assert.strictEqual(onRed, 'var(--super-dark-color)');
+      const onNavy = colorutils.textColorFromBackgroundColor('#111111', 'colibris');
+      assert.strictEqual(onNavy, 'var(--super-light-color)');
+    });
+
+    it('every primary picks a text colour clearing WCAG AA', function () {
+      // The dead-zone regression: for every pure-ish primary, the returned
+      // text colour must produce ≥4.5:1 contrast.
+      const samples = ['#ff0000', '#00ff00', '#0000ff', '#ffff00', '#ff00ff', '#00ffff',
+                       '#800000', '#008000', '#000080', '#808000', '#800080', '#008080'];
+      for (const bg of samples) {
+        const textHex = colorutils.textColorFromBackgroundColor(bg, 'something-else');
+        const textTriple = textHex === '#222'
+            ? colorutils.css2triple('#222222')
+            : colorutils.css2triple('#ffffff');
+        const ratio = colorutils.contrastRatio(colorutils.css2triple(bg), textTriple);
+        assert.ok(ratio >= 4.5, `${bg} → ${textHex} gave only ${ratio.toFixed(2)}:1`);
+      }
     });
   });
 });