feat(ui): warm album art ahead of the scroll

Scrolling the albums grid pops art in: the cards already draw the
smallest adequate tier and are already lazy, so what was left is *when*
the request happens. The grids are virtualized, so the `<img>` — and
therefore the fetch — does not exist until the virtualizer renders its
card, which is about 1000px past the viewport, or two screens on the
reference device.

The issue asks for a larger overscan and that is not available:
`_overhang` is a hard-coded `protected` field on `BaseLayout` with no
configuration surface. So the request is issued ahead of the element
instead. `utils/image-prefetch.ts` warms a bounded window either side
of the rendered range, from `rangeChanged` rather than
`visibilityChanged` — the two report different ranges, and a window
measured from what is *visible* is spent on cards that already exist.

Cover and artist URLs are served under `Cache-Control: immutable`
(content-hashed filenames), so a prefetched image is a cache hit by the
time its card is drawn. The bytes are the browser's; what this holds is
the set of URLs asked for, capped and reported to `__yjCacheStats()`.

Measured on the bulk seed (4 988 albums), ten 2 400px jumps, covers in
the viewport with `naturalWidth === 0`: 254 of 258 blank one frame
after the jump and 214 two frames after, against 117 and 77 with the
prefetch.

Closes #65

frontend/test/components/art-prefetch.test.ts

@@ -0,0 +1,190 @@
+/**
+ * The grids ask for the art below the fold before the card exists
+ * (#65).
+ *
+ * Reported as "scrolling through albums, the art pops in". The cards
+ * already draw the smallest adequate tier and are already
+ * `loading="lazy"`, so what was left is *when*: `<lit-virtualizer>`
+ * renders about 1000px past the viewport and the `<img>` — and
+ * therefore the request — does not exist until it does. On the
+ * reference device that is about two screens.
+ *
+ * These assert the mechanism, since no tier here can photograph a
+ * pop-in: that the rows past the rendered range are requested, that
+ * the request is for the same tier the card will draw, and that the
+ * window has an end — an unbounded prefetch of a 5 000-album library
+ * is the failure this trades against.
+ *
+ * What is *not* asserted here is that a rendered card was never
+ * prefetched. It often was, honestly: the grid lays out more than once
+ * on mount, so a row warmed by the first pass is drawn by the second,
+ * which is the whole point. The rule that a single pass skips its own
+ * rendered range is `image-prefetch.test.ts`'s, where one call can be
+ * looked at on its own.
+ */
+import { describe, expect, it, beforeEach } from 'vitest';
+import type { LitElement } from 'lit';
+
+import '@components/cover-grid/cover-grid';
+import '@components/artists-view/artists-view';
+import { emit, stub, flush, resetHarness } from '@test/support/harness';
+import { Events } from '../../src/events';
+import { fixture, shadowAll } from '@test/support/render';
+import {
+  PREFETCH_AHEAD,
+  imagePrefetched,
+  resetImagePrefetch,
+} from '@utils/image-prefetch';
+
+/** Enough albums that the virtualizer's own window is nowhere near the end. */
+const ALBUMS = Array.from({ length: 400 }, (_, i) => {
+  const n = String(i + 1).padStart(4, '0');
+
+  return {
+    ID: i + 1,
+    Name: `Album ${n}`,
+    ArtistName: 'Aurora Fields',
+    Year: 2020,
+    CoverArtPath: `/covers/${n}.jpg`,
+    CoverArtSmall: `/covers/${n}_sm.jpg`,
+    CoverArtMedium: `/covers/${n}_md.jpg`,
+    CoverArtLarge: `/covers/${n}_lg.jpg`,
+  };
+});
+
+const ARTISTS = Array.from({ length: 400 }, (_, i) => {
+  const n = String(i + 1).padStart(4, '0');
+
+  return {
+    ID: i + 1,
+    Name: `Artist ${n}`,
+    AlbumCount: 2,
+    TrackCount: 9,
+    ImageSmall: `/artists/${n}_sm.jpg`,
+    ImageMedium: `/artists/${n}_md.jpg`,
+    ImageLarge: `/artists/${n}_lg.jpg`,
+  };
+});
+
+/** Give the virtualizer a viewport; a zero-height host renders nothing. */
+function sized(el: HTMLElement): void {
+  el.style.display = 'block';
+  el.style.height = '600px';
+  el.style.width = '900px';
+}
+
+async function settle(el: LitElement): Promise<void> {
+  await flush();
+  await el.updateComplete;
+  await new Promise((r) => setTimeout(r, 200));
+}
+
+/** The `src` of every card the grid actually rendered. */
+function renderedSources(el: LitElement, selector: string): string[] {
+  return shadowAll(el, selector)
+    .map((img) => (img as HTMLImageElement).getAttribute('src') ?? '')
+    .filter(Boolean);
+}
+
+/**
+ * The last index the virtualizer has rendered, read off the cards
+ * rather than counted: the rendered range is what the prefetch window
+ * is measured from, and a count assumes it starts at 0 and has no
+ * gaps.
+ */
+function lastRenderedIndex(el: LitElement, selector: string): number {
+  const indices = shadowAll(el, selector).map((card) =>
+    Number(card.getAttribute('data-index')),
+  );
+
+  return Math.max(...indices);
+}
+
+/**
+ * The tier the cards chose, read off a rendered card rather than
+ * recomputed — the point of the assertion is that the prefetch and the
+ * card agree, so deriving both from the same ladder here would prove
+ * nothing.
+ */
+function tierSuffix(src: string): string {
+  const m = /_(sm|md|lg)\.jpg$/.exec(src);
+
+  return m ? `_${m[1]}` : '';
+}
+
+beforeEach(() => {
+  resetHarness();
+  resetImagePrefetch();
+  localStorage.clear();
+  stub('library.Library.GetAlbums', ALBUMS);
+  stub('library.Library.GetArtists', ARTISTS);
+  stub('library.Library.GetTracks', []);
+  stub('library.Library.GetGenres', []);
+  emit(Events.LibraryScanComplete);
+});
+
+describe('the albums grid warms the covers below the fold', () => {
+  it('asks for the covers past the rendered range, in the tier the card draws', async () => {
+    const el = await fixture<LitElement>('cover-grid');
+
+    sized(el);
+    await settle(el);
+
+    const rendered = renderedSources(el, 'img.cover-image');
+
+    expect(rendered.length).toBeGreaterThan(0);
+
+    const tier = tierSuffix(rendered[0]!);
+    const url = (index: number) =>
+      `/covers/${String(index + 1).padStart(4, '0')}${tier}.jpg`;
+
+    // The grid starts at the top and never scrolls here, so the whole
+    // window lies past the last card drawn.
+    const last = lastRenderedIndex(el, '.album-card');
+
+    expect(imagePrefetched(url(last + 1))).toBe(true);
+    expect(imagePrefetched(url(last + PREFETCH_AHEAD))).toBe(true);
+  });
+
+  it('stops at the end of the window rather than warming the library', async () => {
+    const el = await fixture<LitElement>('cover-grid');
+
+    sized(el);
+    await settle(el);
+
+    const rendered = renderedSources(el, 'img.cover-image');
+    const tier = tierSuffix(rendered[0]!);
+    const url = (index: number) =>
+      `/covers/${String(index + 1).padStart(4, '0')}${tier}.jpg`;
+
+    // Not "exactly `last + PREFETCH_AHEAD`": the grid lays out more
+    // than once on mount and each pass warms a window from wherever
+    // the rendered range was then, so the reachable set is a few
+    // windows wide. The property that matters is that it is a window
+    // at all rather than the library.
+    expect(imagePrefetched(url(399))).toBe(false);
+    expect(window.__yjCacheStats?.()['imagePrefetch']?.entries ?? 0)
+      .toBeLessThan(ALBUMS.length / 2);
+  });
+});
+
+describe('the artists grid warms its avatars the same way', () => {
+  it('asks for the avatars past the rendered range', async () => {
+    const el = await fixture<LitElement>('artists-view');
+
+    sized(el);
+    await settle(el);
+
+    const rendered = renderedSources(el, 'img.avatar-image');
+
+    expect(rendered.length).toBeGreaterThan(0);
+
+    const tier = tierSuffix(rendered[0]!);
+    const last = lastRenderedIndex(el, '.artist-card');
+    const url = (index: number) =>
+      `/artists/${String(index + 1).padStart(4, '0')}${tier}.jpg`;
+
+    expect(imagePrefetched(url(last + 1))).toBe(true);
+    expect(imagePrefetched(url(399))).toBe(false);
+  });
+});

frontend/test/support/pixel.svg

@@ -0,0 +1,2 @@
+<!-- A real, servable image for the prefetch tests: one transparent pixel. -->
+<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"></svg>

frontend/test/utils/image-prefetch.test.ts

@@ -0,0 +1,113 @@
+/**
+ * What the grids ask for ahead of the scroll (#65).
+ *
+ * The virtualizer renders about 1000px past its viewport and nothing
+ * else can be asked for, because the `<img>` does not exist until the
+ * card does — two screens on the reference device, which is a fraction
+ * of a second at speed. `prefetchImageWindow` issues the request
+ * before the element, so the assertions here are about *which* rows
+ * are asked for, that none is asked for twice, and that a request is
+ * really made rather than merely recorded.
+ */
+import { describe, expect, it, beforeEach } from 'vitest';
+
+import {
+  PREFETCH_MEMORY,
+  imagePrefetched,
+  prefetchImage,
+  prefetchImageWindow,
+  resetImagePrefetch,
+} from '@utils/image-prefetch';
+
+/** A hundred cards, each with its own cover URL. */
+const CARDS = Array.from({ length: 100 }, (_, i) => ({ url: `/covers/${i}_sm.jpg` }));
+
+const urlOf = (card: { url: string }) => card.url;
+
+beforeEach(() => {
+  resetImagePrefetch();
+});
+
+describe('warming the images a scroll is about to reach', () => {
+  it('asks for the rows just past the rendered range, and no further', () => {
+    const issued = prefetchImageWindow(CARDS, 40, 50, urlOf, 3);
+
+    // Three past each edge: 51-53 and 37-39.
+    expect(issued).toBe(6);
+    expect(imagePrefetched('/covers/51_sm.jpg')).toBe(true);
+    expect(imagePrefetched('/covers/53_sm.jpg')).toBe(true);
+    expect(imagePrefetched('/covers/54_sm.jpg')).toBe(false);
+    expect(imagePrefetched('/covers/39_sm.jpg')).toBe(true);
+    expect(imagePrefetched('/covers/37_sm.jpg')).toBe(true);
+    expect(imagePrefetched('/covers/36_sm.jpg')).toBe(false);
+  });
+
+  it('leaves the rendered rows alone — they have their own <img>', () => {
+    prefetchImageWindow(CARDS, 40, 50, urlOf, 3);
+
+    expect(imagePrefetched('/covers/45_sm.jpg')).toBe(false);
+  });
+
+  it('asks for nothing twice, so a scroll back over the same rows is free', () => {
+    prefetchImageWindow(CARDS, 40, 50, urlOf, 3);
+
+    expect(prefetchImageWindow(CARDS, 40, 50, urlOf, 3)).toBe(0);
+  });
+
+  it('clamps at both ends of the list', () => {
+    // At the top of a five-item list nothing precedes the range, and
+    // the tail runs out after two.
+    expect(prefetchImageWindow(CARDS.slice(0, 5), 0, 2, urlOf, 10)).toBe(2);
+  });
+
+  it('asks for nothing when the virtualizer reports an empty range', () => {
+    // `visibilityChanged` reports -1/-1 before anything is laid out.
+    expect(prefetchImageWindow(CARDS, -1, -1, urlOf)).toBe(0);
+  });
+
+  it('skips a card that draws a placeholder rather than an image', () => {
+    expect(prefetchImageWindow(CARDS, 40, 50, () => '', 3)).toBe(0);
+  });
+
+  it('really issues the request, rather than only recording it', async () => {
+    // A served file, so the load succeeds and the resource timing entry
+    // is unambiguous; the query string keeps it distinct per run.
+    const url = `/test/support/pixel.svg?prefetch=${Date.now()}`;
+    const href = new URL(url, location.href).href;
+
+    expect(prefetchImage(url)).toBe(true);
+
+    for (let i = 0; i < 100; i++) {
+      if (performance.getEntriesByName(href).length > 0) break;
+
+      await new Promise((r) => setTimeout(r, 20));
+    }
+
+    expect(performance.getEntriesByName(href)).toHaveLength(1);
+    expect(prefetchImage(url)).toBe(false);
+    expect(performance.getEntriesByName(href)).toHaveLength(1);
+  });
+
+  it('reports what it is holding, with its cap, to the cache stats', () => {
+    prefetchImageWindow(CARDS, 40, 50, urlOf, 3);
+
+    const stat = window.__yjCacheStats?.()['imagePrefetch'];
+
+    expect(stat).toBeTruthy();
+    expect(stat!.entries).toBe(6);
+    expect(stat!.limit).toBe(PREFETCH_MEMORY);
+    // It holds URLs, not images — the bytes are the browser's cache.
+    expect(stat!.chars).toBe(6 * '/covers/51_sm.jpg'.length);
+  });
+
+  it('keeps its record bounded, so a 50 000-album scroll cannot grow it', () => {
+    const many = Array.from(
+      { length: PREFETCH_MEMORY * 2 },
+      (_, i) => ({ url: `/covers/bulk-${i}_sm.jpg` }),
+    );
+
+    prefetchImageWindow(many, 0, 0, urlOf, many.length);
+
+    expect(window.__yjCacheStats?.()['imagePrefetch']?.entries).toBe(PREFETCH_MEMORY);
+  });
+});