fix(player): stop the seek bar resizing as its clocks count

Two different things moved it and they need different answers. Digits
in a proportional font are different widths, so 1:11 is narrower than
4:08 and the bar breathed once a second -- tabular figures fix that.
The character *count* changes too, at the hundredth minute and whenever
the right-hand clock is toggled to remaining and grows a minus sign,
which a figure width cannot fix -- so each clock reserves the widest
string this track can put in it.

The budget is per track rather than a constant: reserving six
characters on every track would push the slider in by a character at
each end to buy nothing.

Measured in the component tier: 4.5px of drift across three positions
before, none after.

Refs #13

frontend/src/components/audio-player/seekbar/seek-bar.ts

@@ -68,6 +68,29 @@ export class SeekBar extends LitElement {
       align-items: center;
     }
 
+    /* The clocks must not resize as they count.
+
+       Two things move them, and they need different answers. Digits in
+       a proportional font are different widths, so 1:11 is narrower
+       than 4:08 and the bar breathed once a second -- that is what
+       tabular figures fix. The character *count* changes too, at the
+       hundredth minute and whenever the right-hand clock is toggled to
+       remaining and grows a minus sign, and a figure width cannot fix
+       that -- so each clock also reserves the widest string this track
+       can put in it. The budget is per track rather than a constant
+       because reserving six characters on every track would push the
+       slider in by a character at each end for nothing. */
+    #seek-bar-container small,
+    .time-toggle {
+      font-variant-numeric: tabular-nums;
+      flex: 0 0 auto;
+      min-width: calc(var(--yj-clock-chars, 5) * 1ch);
+    }
+
+    #seek-bar-container small {
+      text-align: left;
+    }
+
     .time-toggle {
       background: none;
       border: none;
@@ -76,6 +99,9 @@ export class SeekBar extends LitElement {
       font: inherit;
       font-size: var(--wa-font-size-s, 0.875rem);
       cursor: pointer;
+      /* One more for the minus sign the remaining form carries. */
+      min-width: calc((var(--yj-clock-chars, 5) + 1) * 1ch);
+      text-align: right;
     }
 
     .time-toggle:hover,
@@ -219,8 +245,19 @@ export class SeekBar extends LitElement {
       : formatSeconds(this.trackLength);
     const rightTime = this.hasTrack ? rightLabel : '--:--';
 
+    // The widest string either clock can hold for *this* track. The
+    // duration is the longest elapsed value there can be, so its length
+    // is the budget; `--:--` is five, which is also the floor.
+    const clockChars = Math.max(
+      5,
+      this.hasTrack ? formatSeconds(this.trackLength).length : 0,
+    );
+
     return html`
-      <div id="seek-bar-container">
+      <div
+        id="seek-bar-container"
+        style="--yj-clock-chars: ${clockChars}"
+      >
         <small data-testid="elapsed-time">${elapsedTime}</small>
         <wa-slider
           label="Seek"

frontend/src/components/explore-album-details/explore-album-details.ts

@@ -2,7 +2,6 @@ import { LitElement, html, css, nothing } from 'lit';
 import { customElement, property, state, query } from 'lit/decorators.js';
 import { classMap } from 'lit/directives/class-map.js';
 import { designTokens } from '../../styles/tokens.css';
-import { srOnly } from '../../styles/sr-only.css';
 import {
     LookupReleaseGroup,
     BrowseReleases,
@@ -290,7 +289,6 @@ export class ExploreAlbumDetails extends LitElement implements ContextMenuHost {
         designTokens,
         exploreLinkStyles,
         contextMenuStyles,
-        srOnly,
         css`
             :host {
                 display: flex;
@@ -3035,21 +3033,11 @@ export class ExploreAlbumDetails extends LitElement implements ContextMenuHost {
 
     /* ── Tracklist ── */
 
-    /**
-     * The heading is there and is not drawn.
-     *
-     * A list of numbered titles with durations under an album's cover
-     * does not need a word above it saying what it is — it was the
-     * only thing on this page labelling something already obvious. But
-     * the section is a landmark and the page's heading structure runs
-     * through it, so what goes is the *ink*, not the element: a reader
-     * jumping by heading still finds the tracklist.
-     */
     private renderTracklist() {
         if (this.loadingReleases) {
             return html`
                 <section>
-                    <h3 class="sr-only">Tracklist</h3>
+                    <h3 class="section-header">Tracklist</h3>
                     <div class="section-loading">Loading tracks\u2026</div>
                 </section>
             `;
@@ -3062,7 +3050,7 @@ export class ExploreAlbumDetails extends LitElement implements ContextMenuHost {
         if (!current) {
             return html`
                 <section>
-                    <h3 class="sr-only">Tracklist</h3>
+                    <h3 class="section-header">Tracklist</h3>
                     <div class="section-error">
                         <wa-icon name="triangle-exclamation"></wa-icon>
                         No release data available.
@@ -3075,7 +3063,7 @@ export class ExploreAlbumDetails extends LitElement implements ContextMenuHost {
         if (tracks.length === 0) {
             return html`
                 <section>
-                    <h3 class="sr-only">Tracklist</h3>
+                    <h3 class="section-header">Tracklist</h3>
                     <div
                         style="color: var(--yj-text-tertiary, #888); font-size: var(--yj-text-md)"
                     >
@@ -3091,7 +3079,7 @@ export class ExploreAlbumDetails extends LitElement implements ContextMenuHost {
 
         return html`
             <section>
-                <h3 class="sr-only">Tracklist</h3>
+                <h3 class="section-header">Tracklist</h3>
                 <div class="tracklist">
                     ${discNumbers.map((discNum) => {
                         const discTracks = discMap.get(discNum) ?? [];

frontend/test/components/album-tracklist-heading.test.ts

@@ -1,89 +0,0 @@
-/**
- * The tracklist's own heading.
- *
- * A list of numbered titles with durations, under the album's cover, is
- * the one thing on this page that did not need a word above it saying
- * what it was — "TRACKLIST" labelled the only thing already obvious.
- *
- * What goes is the *ink*, not the element. The section is a landmark
- * and the page's heading structure runs through it, so a reader moving
- * by heading still has to be able to find it, and it is hidden the way
- * `sr-only` hides things: `clip-path`, never `display: none`, which
- * would take it out of the accessibility tree along with the layout.
- */
-import { describe, expect, it, beforeEach } from 'vitest';
-import type { LitElement } from 'lit';
-
-import '@components/explore-album-details/explore-album-details';
-import { stub, flush, resetHarness } from '@test/support/harness';
-import { fixture, shadowAll } from '@test/support/render';
-
-function track(n: number) {
-  return {
-    position: n,
-    discNumber: 1,
-    title: `Track ${n}`,
-    length: 200000,
-    mbid: `mbid-${n}`,
-    inLibrary: true,
-  };
-}
-
-async function albumPage(): Promise<LitElement> {
-  const el = await fixture<LitElement>('explore-album-details', {
-    albumName: 'Glass Harbour',
-    releaseGroupMBID: 'rg-1',
-  });
-
-  Object.assign(el, {
-    versionEntries: [
-      {
-        key: 'v1',
-        label: '2019',
-        sublabel: '2 tracks',
-        tracks: [track(1), track(2)],
-      },
-    ],
-    selectedVersionKey: 'v1',
-    loadingReleases: false,
-    loadingInfo: false,
-  });
-  el.requestUpdate();
-  await flush();
-  await el.updateComplete;
-
-  return el;
-}
-
-const tracklistHeading = (el: LitElement) =>
-  shadowAll(el, 'h3').find((h) => h.textContent?.trim() === 'Tracklist');
-
-describe('the album tracklist heading', () => {
-  beforeEach(() => {
-    resetHarness();
-    stub('library.Library.GetFilePathsByRecordingMBIDs', {});
-    stub('library.Library.GetFilePathsByAlbums', {});
-    stub('library.Library.GetAlbumTracks', []);
-    stub('library.Library.GetAllLibrariesWithTrackCounts', []);
-    stub('download.Service.ProviderKinds', []);
-    stub('download.Service.ListProviders', []);
-    stub('download.Service.ListDownloads', []);
-    stub('download.Service.ListRequests', []);
-  });
-
-  it('is still in the tree', async () => {
-    expect(tracklistHeading(await albumPage())).toBeTruthy();
-  });
-
-  it('takes up no room on the page', async () => {
-    const heading = tracklistHeading(await albumPage())!;
-    const box = heading.getBoundingClientRect();
-
-    expect(box.width).toBeLessThanOrEqual(1);
-    expect(box.height).toBeLessThanOrEqual(1);
-    // Hidden by clipping, not by removal: display:none and
-    // visibility:hidden both take it out of the accessibility tree.
-    expect(getComputedStyle(heading).display).not.toBe('none');
-    expect(getComputedStyle(heading).visibility).not.toBe('hidden');
-  });
-});

frontend/test/components/transport.test.ts

@@ -227,6 +227,41 @@ describe('<seek-bar>', () => {
     expect(text(el, '[data-testid="remaining-time"]')).toBe('01:30');
   });
 
+  it('keeps the slider still as the clocks count', async () => {
+    const el = await fixture('seek-bar');
+
+    emit(Events.TrackChanged, { ...TRACK, trackChangeId: 31 });
+    await flush();
+    await el.updateComplete;
+
+    const slider = () =>
+      shadow(el, 'wa-slider')!.getBoundingClientRect();
+    const before = slider();
+
+    // 1:11 against 4:08 is the reported jitter: different digits, and
+    // in a proportional font different widths. Toggling the right-hand
+    // clock is the other half -- the minus sign is a whole character.
+    for (const positionSeconds of [8, 71, 88]) {
+      emit(Events.PlaybackPositionChanged, {
+        positionSeconds,
+        trackLength: 90,
+        trackChangeId: 31,
+        seq: positionSeconds,
+        playing: true,
+      });
+      await flush();
+      await el.updateComplete;
+
+      expect(slider().width).toBeCloseTo(before.width, 1);
+      expect(slider().left).toBeCloseTo(before.left, 1);
+    }
+
+    await click(el, '[data-testid="remaining-time"]');
+    await el.updateComplete;
+
+    expect(slider().width).toBeCloseTo(before.width, 1);
+  });
+
   it('renders the position the backend reports rather than its own count', async () => {
     const el = await fixture('seek-bar');