Fix: skip hex nested in color-mix when measuring gradient contrast (#578)

parseGradientColors treated #000 inside color-mix() as a stop, so low-contrast scored text against phantom black. Prepared with AI assistance.

Co-authored-by: Cursor <cursoragent@cursor.com>

cli/engine/detect-antipatterns-browser.js

@@ -773,14 +773,22 @@ function extractColorFunctionTokens(value) {
 function parseGradientColors(bgImage) {
   if (!bgImage || !bgImage.includes('gradient')) return [];
   const colors = [];
+  const tokenSpans = [];
+  let from = 0;
   // Stops arrive in whatever syntax the author wrote and the browser kept.
   // A dark ground painted as `linear-gradient(oklch(...), oklch(...))` used
   // to read as a gradient with no stops at all.
   for (const token of extractColorFunctionTokens(bgImage)) {
+    const start = bgImage.indexOf(token, from);
+    if (start < 0) break;
+    tokenSpans.push({ start, end: start + token.length });
+    from = start + token.length;
     const c = parseAnyColor(token);
     if (c) colors.push(c);
   }
   for (const m of bgImage.matchAll(/#([0-9a-f]{6}|[0-9a-f]{3})\b/gi)) {
+    // Nested hex inside color-mix is an ingredient, not a stop (issue #578).
+    if (tokenSpans.some(s => m.index >= s.start && m.index < s.end)) continue;
     const h = m[1];
     if (h.length === 6) {
       colors.push({ r: parseInt(h.slice(0,2),16), g: parseInt(h.slice(2,4),16), b: parseInt(h.slice(4,6),16), a: 1 });

cli/engine/shared/color.mjs

@@ -103,14 +103,22 @@ function extractColorFunctionTokens(value) {
 function parseGradientColors(bgImage) {
   if (!bgImage || !bgImage.includes('gradient')) return [];
   const colors = [];
+  const tokenSpans = [];
+  let from = 0;
   // Stops arrive in whatever syntax the author wrote and the browser kept.
   // A dark ground painted as `linear-gradient(oklch(...), oklch(...))` used
   // to read as a gradient with no stops at all.
   for (const token of extractColorFunctionTokens(bgImage)) {
+    const start = bgImage.indexOf(token, from);
+    if (start < 0) break;
+    tokenSpans.push({ start, end: start + token.length });
+    from = start + token.length;
     const c = parseAnyColor(token);
     if (c) colors.push(c);
   }
   for (const m of bgImage.matchAll(/#([0-9a-f]{6}|[0-9a-f]{3})\b/gi)) {
+    // Nested hex inside color-mix is an ingredient, not a stop (issue #578).
+    if (tokenSpans.some(s => m.index >= s.start && m.index < s.end)) continue;
     const h = m[1];
     if (h.length === 6) {
       colors.push({ r: parseInt(h.slice(0,2),16), g: parseInt(h.slice(2,4),16), b: parseInt(h.slice(4,6),16), a: 1 });

tests/detect-antipatterns-fixtures.test.mjs

@@ -272,6 +272,31 @@ describe('detectHtml — static HTML/CSS fixtures', () => {
     );
   });
 
+  it('color: nested #000 inside color-mix must not become on #000000', async () => {
+    const f = await detectHtml(path.join(FIXTURES, 'color.html'));
+    const light = f.filter(r =>
+      (r.antipattern === 'low-contrast' || r.antipattern === 'gray-on-color') &&
+      /#f7f3ea/i.test(r.snippet || '')
+    );
+    assert.equal(
+      light.length, 0,
+      `light text on the mixed green must not flag: ${light.map(r => r.snippet).join('; ')}`,
+    );
+    const leaked = f.filter(r => /#3d2418 on #000000/i.test(r.snippet || ''));
+    assert.equal(
+      leaked.length, 0,
+      `nested #000 must not become on #000000: ${leaked.map(r => r.snippet).join('; ')}`,
+    );
+    assert.ok(
+      f.some(r =>
+        r.antipattern === 'low-contrast' &&
+        /#3d2418/i.test(r.snippet || '') &&
+        /#17372d|#295344/i.test(r.snippet || '')
+      ),
+      'dark ink on the mixed stop should flag against the mix, not phantom black',
+    );
+  });
+
   it('color: white text on background-image url() ancestor is not flagged as low-contrast', async () => {
     const f = await detectHtml(path.join(FIXTURES, 'color.html'));
     // The pass column has white text on a div with background-image: url().

tests/detect-antipatterns.test.js

@@ -1599,6 +1599,32 @@ describe('hover contrast + color-mix', () => {
     expect(stops).toHaveLength(2);
   });
 
+  test('parseGradientColors resolves color-mix stops without leaking nested hex', () => {
+    const stops = parseGradientColors('linear-gradient(135deg, color-mix(in srgb, #2d5a4a 92%, #000), color-mix(in srgb, #1a3d32 90%, #000))');
+    expect(stops).toHaveLength(2);
+    expect(stops[0]).toEqual({ r: 41, g: 83, b: 68, a: 1 });
+    expect(stops[1]).toEqual({ r: 23, g: 55, b: 45, a: 1 });
+  });
+
+  test('parseGradientColors does not leak nested hex when color-mix has var()', () => {
+    const stops = parseGradientColors('linear-gradient(135deg, color-mix(in srgb, var(--brand) 92%, #000), color-mix(in srgb, var(--brand-deep) 90%, #000))');
+    expect(stops).toEqual([]);
+  });
+
+  test('parseGradientColors still collects sibling bare hex stops beside color-mix', () => {
+    const stops = parseGradientColors('linear-gradient(color-mix(in srgb, #2d5a4a 92%, #000), #ffffff)');
+    expect(stops).toHaveLength(2);
+    expect(stops[0]).toEqual({ r: 41, g: 83, b: 68, a: 1 });
+    expect(stops[1]).toEqual({ r: 255, g: 255, b: 255, a: 1 });
+  });
+
+  test('parseGradientColors still reads bare hex gradient stops', () => {
+    const stops = parseGradientColors('linear-gradient(#2d5a4a, #000)');
+    expect(stops).toHaveLength(2);
+    expect(stops[0]).toEqual({ r: 45, g: 90, b: 74, a: 1 });
+    expect(stops[1]).toEqual({ r: 0, g: 0, b: 0, a: 1 });
+  });
+
   test('checkHoverContrast flags a failing hover pair on a styled control', () => {
     const f = checkHoverContrast({
       tag: 'a',

tests/fixtures/antipatterns/color.html

@@ -45,11 +45,14 @@
     .mix-dark-wrap { background: #0f0f11; padding: 16px; }
     .mix-glow { background: linear-gradient(160deg, color-mix(in oklab, oklch(90% 0.02 95) 16%, transparent) 0%, #141419 65%); padding: 20px; }
     .mix-glow p { color: #ded9cf; font-size: 16px; }
+    /* issue #578 — #000 inside color-mix is an ingredient; white-ish text on
+       the mixed dark green must not be scored against phantom black. */
+    .mix-hex-brand { background: linear-gradient(135deg, color-mix(in srgb, var(--mix-hex-brand) 92%, #000), color-mix(in srgb, var(--mix-hex-brand-deep) 90%, #000)); padding: 20px; }
     /* currentcolor surface: background-color paints with the element's own
        text color, which is itself a var() token here. jsdom hands both
        through verbatim, so the walk must resolve the token via the
        custom-prop map instead of abstaining on a knowable surface. */
-    :root { --fixture-bone: #e8e2d6; }
+    :root { --fixture-bone: #e8e2d6; --mix-hex-brand: #2d5a4a; --mix-hex-brand-deep: #1a3d32; }
     .currentcolor-surface { background-color: currentcolor; color: var(--fixture-bone); padding: 14px 16px; border-radius: 10px; margin-bottom: 10px; }
     .currentcolor-low-text { color: #cfc9bd; font-size: 14px; }
     .currentcolor-good-text { color: #3a352c; font-size: 14px; }
@@ -133,6 +136,13 @@
         <p>Purple-to-indigo gradient</p>
       </div>
 
+      <h3>color-mix nested hex must not report phantom black</h3>
+      <!-- Dark ink on the mixed green is a real fail against #17372d. The
+           leaked-#000 extractor used to report it as on #000000 instead. -->
+      <div class="mix-hex-brand" data-test="mix-hex-brand-dark">
+        <p style="color: #3d2418; font-size: 16px;">Dark ink on a mixed green stop must not report on #000000</p>
+      </div>
+
       <h3>currentcolor surface via var() token</h3>
       <!-- background-color: currentcolor with color: var(--fixture-bone).
            The surface is knowable (bone #e8e2d6), so the faint text on it is
@@ -248,6 +258,11 @@
         </div>
       </div>
 
+      <h3>color-mix nested hex is not a surface</h3>
+      <div class="mix-hex-brand" data-test="mix-hex-brand">
+        <p style="color: #f7f3ea; font-size: 16px;">WhatsApp-style light text on a mixed dark green gradient stays readable</p>
+      </div>
+
       <h3>currentcolor surface with good contrast</h3>
       <div class="currentcolor-surface" data-test="currentcolor-good">
         <p class="currentcolor-good-text">Dark ink text on a bone currentcolor surface</p>