fix(a11y): mark the playing row with a shape, not only a colour

a11y.22, WCAG 1.4.1: `.track-row.active` was a background tint and a
text colour, and the row markup carried no aria-current either — so a
colour-blind user could not find the playing row and AT had no signal
at all. The queue panel had aria-current from Phase 1 and the same
colour-only visual.

A triangle drawn in each row's own left padding by `::before`. It is a
shape that is present or absent, and it costs no layout: track-list's
grid columns are computed from the host width, so a marker in the flow
would move every cell on the playing row and nothing else.

Both directions are asserted in both tiers. A marker that renders on
every row satisfies "the playing row has one" for free, which is this
plan's oldest rule.

And one thing the reproduction found: a track started from the *list*
leaves the queue's currentIndex at -1, so the panel has no current row
in that flow at all. Pre-existing, and the reason this looked broken
the first time it was checked in the running app.

e2e/specs/playback.spec.ts

@@ -141,3 +141,38 @@ test.describe('queue', () => {
     await expect(shuffle).toHaveAttribute('aria-pressed', 'false');
   });
 });
+
+test.describe('the playing row is findable without colour vision', () => {
+  test('the queue marks its current row with a shape and aria-current', async ({
+    app,
+  }) => {
+    await app.getByTestId('nav-tracks').click();
+    await app.getByTestId('track-row').first().dblclick();
+
+    const queueToggle = app.locator('#queue-button');
+
+    await queueToggle.click();
+
+    const row = app.getByTestId('queue-row').first();
+
+    await expect(row).toBeVisible();
+
+    // Played *from the queue*, because a track started from the list
+    // leaves `currentIndex` at -1 — so the panel has no current row at
+    // all in that flow, which is what made this marker look broken the
+    // first time it was checked.
+    await row.dblclick();
+    await expect(row).toHaveAttribute('aria-current', 'true');
+
+    const marker = await row.evaluate(
+      (el) => getComputedStyle(el, '::before').borderLeftWidth,
+    );
+
+    // `a11y.22`: a background tint and a text colour were the only two
+    // signals, and both are hue (WCAG 1.4.1).
+    expect(parseFloat(marker)).toBeGreaterThan(0);
+
+    await queueToggle.click();
+    await expect(app.getByTestId('queue-row')).toHaveCount(0);
+  });
+});

frontend/src/components/queue-panel/queue-panel.ts

@@ -378,6 +378,22 @@ export class QueuePanel
             color: var(--yj-accent-text, #ffd43b);
         }
 
+        /* a11y.22, the same rule as track-list one panel over: the
+           playing row was a tint and a text colour and nothing else.
+           The triangle lives in the row's own 16px left padding, so it
+           is a shape that is present or absent and the flex row does
+           not move. */
+        .track-item.active::before {
+            content: '';
+            position: absolute;
+            left: 5px;
+            top: 50%;
+            transform: translateY(-50%);
+            border-left: 5px solid var(--yj-accent-text, #ffd43b);
+            border-top: 4px solid transparent;
+            border-bottom: 4px solid transparent;
+        }
+
         .track-art {
             width: 32px;
             height: 32px;

frontend/src/components/track-list/track-list.ts

@@ -1002,6 +1002,7 @@ export class TrackList
     }
 
     .track-row {
+      position: relative;
       display: grid;
       grid-template-columns: var(--grid-cols);
       font-size: var(--yj-text-sm);
@@ -1042,6 +1043,24 @@ export class TrackList
       color: var(--yj-accent-text, #ffd43b);
     }
 
+    /* a11y.22: the playing row was a background tint and a text colour
+       and nothing else, so a colour-blind user could not find it
+       (WCAG 1.4.1). A triangle drawn in the row's own 8px left padding
+       is a *shape* that is present or absent, and it costs no layout:
+       the grid columns are computed from the host width and every one
+       of them would have had to move for a marker in the flow. Sized
+       to that padding rather than to the glyph a font would give. */
+    .track-row.active::before {
+      content: '';
+      position: absolute;
+      left: 1px;
+      top: 50%;
+      transform: translateY(-50%);
+      border-left: 5px solid currentColor;
+      border-top: 4px solid transparent;
+      border-bottom: 4px solid transparent;
+    }
+
     .track-row.selected.active {
       background-color: var(--yj-selection-bg, rgba(100, 160, 255, 0.15));
     }
@@ -1808,6 +1827,7 @@ export class TrackList
         role="row"
         aria-rowindex=${index + 1}
         aria-selected=${selected}
+        aria-current=${active ? 'true' : 'false'}
         tabindex=${index === this.focusedIndex ? 0 : -1}
         draggable="true"
         data-index=${index}

frontend/test/components/aria-tail.test.ts

@@ -235,3 +235,87 @@ describe('a clipped value is readable somewhere', () => {
     expect(shadow(el, '.secondary')?.getAttribute('title')).toBeTruthy();
   });
 });
+
+describe('the playing row is more than a colour', () => {
+  beforeEach(async () => {
+    resetHarness();
+    searchStore.setTerm('');
+    stub('library.Library.GetAllTracks', TRACKS);
+    stub('library.Library.GetAllAlbums', []);
+    emit(Events.LibraryScanComplete);
+  });
+
+  /** The row the player says it is on, once the list has settled. */
+  async function listWithPlayingTrack(): Promise<LitElement> {
+    const el = await fixture<LitElement>('track-list');
+
+    sized(el);
+    await settle(el);
+
+    emit(Events.TrackChanged, {
+      fileName: 'a.mp3',
+      filePath: '/m/a.mp3',
+      trackLength: 4,
+      seekPosition: 0,
+      state: 'playing',
+      title: 'Departure',
+      artist: 'Aurora Fields',
+      album: 'Glass Harbour',
+      coverArt: '',
+      coverArtSmall: '',
+      coverArtMedium: '',
+      coverArtLarge: '',
+      trackChangeId: 1,
+      artistMbid: '',
+      releaseGroupMbid: '',
+      recordingMbid: '',
+    });
+    await settle(el);
+
+    return el;
+  }
+
+  it('marks exactly one row aria-current', async () => {
+    const el = await listWithPlayingTrack();
+
+    const current = shadowAll(el, '.track-row[aria-current="true"]');
+
+    // The positive case first: a guard that marks nothing passes
+    // "at most one" for free.
+    expect(current).toHaveLength(1);
+    expect(current[0]!.classList.contains('active')).toBe(true);
+  });
+
+  it('draws a marker that is a shape, not a hue', async () => {
+    const el = await listWithPlayingTrack();
+
+    const active = shadow(el, '.track-row.active');
+    const other = shadowAll(el, '.track-row:not(.active)')[0];
+
+    expect(active, 'no active row rendered').toBeTruthy();
+
+    // `::before` has no box unless it has content, so a width of zero
+    // on the inactive row is the assertion that the marker is *absent*
+    // there — which is the half that makes the present one mean
+    // something (a11y.22, WCAG 1.4.1).
+    const width = (el: Element) =>
+      parseFloat(getComputedStyle(el, '::before').borderLeftWidth) || 0;
+
+    expect(width(active!)).toBeGreaterThan(0);
+    expect(width(other!)).toBe(0);
+  });
+
+  it('does not move the row it marks', async () => {
+    const el = await listWithPlayingTrack();
+
+    const rows = shadowAll(el, '.track-row');
+    const lefts = rows.map(
+      (r) => r.querySelector('.cell')!.getBoundingClientRect().left,
+    );
+
+    // The marker lives in the row's own padding: the grid columns are
+    // computed from the host width, so anything in the flow would move
+    // every cell on the playing row and nothing else.
+    expect(new Set(lefts.map(Math.round)).size).toBe(1);
+  });
+});

frontend/test/components/queue-reorder.test.ts

@@ -194,6 +194,21 @@ describe('a queue row says which track its controls act on', () => {
     ]);
   });
 
+  it('marks the playing row with a shape, not only a colour', async () => {
+    const el = await panelWithQueue();
+
+    const rows = shadowAll(el, '.track-item');
+    const width = (r: Element) =>
+      parseFloat(getComputedStyle(r, '::before').borderLeftWidth) || 0;
+
+    // `a11y.22`, the same rule as track-list one panel over. Both
+    // directions: absent on the rows that are not playing is what
+    // makes the present one mean anything.
+    expect(rows[0]!.getAttribute('aria-current')).toBe('true');
+    expect(width(rows[0]!)).toBeGreaterThan(0);
+    expect(width(rows[1]!)).toBe(0);
+  });
+
   it('gives the title and artist a tooltip, since the panel is resizable', async () => {
     const el = await panelWithQueue();