fix(ui): the phone's nav sheet says when it scrolls

Since #71 the phone's "More" is a bottom sheet, and at the reference
viewport it does not fit: measured at 424x439 with the seed's eight
destinations, the body is scrollHeight 412 against clientHeight 373, so
39px is below a fold nothing announces. Where the cut lands on a row
boundary the sheet ends in a clean edge that reads as the end of the
list, which is what #207 fixed one sheet over.

The rule is that sheet's, not a second answer to the same question:
#207's two background layers move into styles/sheet-scroll.css.ts and
both sheets adopt them, with the colour left to each host as
--yj-sheet-surface. The nav sheet paints the sidebar's --yj-bg-surface
and the context sheet the menus' --yj-bg-elevated, so a shared rule that
hard-coded either would draw that seam across the other one.

The half that makes it visible is that nothing inside the sheet may
repaint the surface. These are layers on the scroller, and app-sidebar's
host carries the same grey -- in the shell its own background, in the
sheet a second opaque copy of the sheet's, over the fade. With the
fragment adopted and that rule missing, the running app measured a flat
52,58,64 to the bottom edge with 39px still below: the defect unchanged,
with every assertion about background-attachment passing. menu-surface
already meets it from the other side, where the sheet's panel is
background-color: transparent.

Closes #210

.planning/NOTES.md

@@ -5029,3 +5029,52 @@ Findings offer it for the 300ms tap delay; this app's viewport is
 the stated benefit is not there to win. What it would change is the
 gesture stack #63 tuned by measurement on the device (`pan-y` plus a
 non-passive `preventDefault`), and that is not measurable from here.
+
+## Art pop-in is measurable in a browser, if you count frames rather than milliseconds (measured 2026-08-24)
+
+#65 is an Android report ("scrolling through albums, the art pops in")
+and the desktop harness can measure it, which was not obvious: the
+first attempt waited 220 ms after each scroll jump and found **zero**
+blank covers on either build. The metric only discriminates at one and
+two animation frames after the jump, which is where a pop-in actually
+lives.
+
+Protocol, on `make dev-headless SEED=bulk` (4 988 albums), ten
+2 400px jumps of `.grid-scroll-container`, counting covers whose rect
+intersects the viewport with `naturalWidth === 0`:
+
+| build | blank at frame 1 | at frame 2 | at 50 ms |
+|---|---|---|---|
+| `main` | 254 / 258 | 214 / 258 | 0 |
+| `main`, second run | 254 / 258 | 190 / 258 | 0 |
+| prefetch | 117 / 258 | 77 / 258 | 0 |
+| prefetch, second run | 118 / 258 | 96 / 258 | 0 |
+
+Two things this protocol gets wrong if repeated carelessly. **A second
+run in the same browser session measures the HTTP cache**, not the
+build — the skill already warns about this for `make perf`, and it
+applies to any image measurement; every row above is a fresh
+`playwright-cli close` + `open`. And **the frontend is embedded**, so
+comparing builds is a `git stash` *and* a rebuild, not a stash.
+
+**The bulk library's covers are 300x300 and ~3.7 kB**, which is why
+both builds are clean by 50 ms here and why the phone's number cannot
+be inferred from this one — same caveat the skill already records
+about full-size artwork.
+
+**`rangeChanged` and `visibilityChanged` are different ranges**, and
+the difference is the whole of this fix's value.
+`@lit-labs/virtualizer` reports `_first`/`_last` (rendered, including
+the ~1000px overhang) on the former and `_firstVisible`/`_lastVisible`
+on the latter. Both grids listen to `visibilityChanged` for scroll
+persistence, which wants the visible range and is correct; a prefetch
+window measured from it lands mostly on cards that already exist.
+Anchored there, the component test could see only one row past the
+last rendered card.
+
+**`_overhang` is not configurable.** It is a `protected` field set to
+1000 in `BaseLayout` and read by every layout; there is no option on
+`grid()`/`flow()` and no property on the element. The issue's Direction
+("ask the virtualizer for a larger overscan") is therefore not
+available without patching a private, which is why the request is
+issued ahead of the element instead.

CLAUDE.md

@@ -1470,6 +1470,28 @@ live**: a scrim over a menu item is that item's text surface, and the
 14px spends its weight below the last legible label, measured at 9.9:1
 on the light ramp, whose `bgElevated` is `#e9ecef`.
 
+**And the phone has two sheets, so that rule is one file both read**
+(#210). `bottom-nav`'s "More" is capped at the same 85vh and overflows
+for the same reason — measured at 424x439 with eight destinations,
+`scrollHeight` 412 against `clientHeight` 373, and eleven items at 48px
+would be 528, since #25 makes the count the user's. So the two layers
+live in `styles/sheet-scroll.css.ts` and each host says only what is
+local to it: the colour, handed over as `--yj-sheet-surface` on the same
+box, because the nav sheet paints the sidebar's `--yj-bg-surface` and
+the context sheet the menus' `--yj-bg-elevated` — a shared rule that
+hard-coded either would draw that seam across the other one.
+
+The half that is not the fade is what makes it visible: **nothing inside
+the sheet may repaint the surface**, because these are background layers
+on the scroller and an opaque child covers them. `menu-surface` already
+had it from the other side (`.context-menu-panel[data-sheet]` is
+`background-color: transparent`); `app-sidebar`'s host paints
+`--yj-bg-surface`, which in the shell is its own background and in the
+sheet is a second copy of the sheet's, so `bottom-nav` turns it off.
+Measured at 424x439 with the fade adopted and that rule missing: a flat
+52,58,64 to the bottom edge with 39px still below, which is the defect
+unchanged and every assertion about `background-attachment` passing.
+
 **The playlist submenu is a sheet too, and it had to be.** It is a
 `placement="right-start"` flyout, and making the menu full-width moved
 its anchor — measured at x −182 to 0, entirely off-screen, so "Add to
@@ -3403,6 +3425,46 @@ rather than searching it — the store replaces that array when its
 contents change and shares the unchanged members, which is the same
 signal `track-list`'s memoized caches key on.
 
+**And the right tier arriving late still reads as no art at all**, so
+the two grids ask for it before the card exists (#65).
+`utils/image-prefetch.ts` warms the images a scroll is about to reach,
+from `cover-grid`'s and `artists-view`'s virtualizers. Measured on the
+50 000-track bulk seed over ten 2 400px jumps: of 258 covers arriving
+in view, **254 were still blank one frame later and 214 two frames
+later**; with the prefetch, 117 and 77. Both builds are clean by 50 ms
+on a desktop with 3.7 kB fixture covers, which is where the reference
+device's slower engine and 27 kB covers spend their pop-in.
+
+Four things about it are load-bearing.
+
+**The overscan the obvious fix asks for does not exist.**
+`@lit-labs/virtualizer`'s `_overhang` is a hard-coded 1000px
+`protected` field on `BaseLayout` with no configuration surface, so
+raising it means monkey-patching a private. 1000px is about two
+screens on a 439px viewport, and the *image* cannot be requested until
+the card it lives in is rendered — which is what this asks for
+instead.
+
+**It hangs off `rangeChanged`, not `visibilityChanged`.** Those report
+different ranges: visibility is what is on screen, and the virtualizer
+has already rendered that 1000px past it. Anchored to the visible
+range the window is spent on cards that already exist and have already
+asked for their own art — measured as the difference between the
+prefetch reaching one row past the last card and reaching a full
+window past it.
+
+**It is not the `LRUMap` path, and saying so is the bound.** That
+ceiling holds Explore's base64 data URLs in JS; a library cover is a
+plain URL under `Cache-Control: immutable` (the filenames are content
+hashes), so what retains the bytes is the browser's own cache. What
+this module retains is the *set of URLs already asked for*, capped at
+512 and reported to `window.__yjCacheStats()` — 497 entries and 15 407
+chars after the run above.
+
+**The prefetch asks for what the card will draw.** `artists-view`'s
+tier ladder moved into `artistAvatarURL()` so the two cannot disagree;
+a second copy would be a warm cache for a tier nothing renders.
+
 **The same rule, on the selection path, was the worst stall in the
 app.** Five components turned selected file paths back into tracks with
 `filePaths.map(fp => tracks.find(…))`, so "Select all → Edit tags" at

frontend/src/components/artists-view/artists-view.ts

@@ -7,6 +7,7 @@ import {
 import '@lit-labs/virtualizer';
 import type {
     LitVirtualizer,
+    RangeChangedEvent,
     VisibilityChangedEvent,
 } from '@lit-labs/virtualizer';
 import { grid } from '@lit-labs/virtualizer/layouts/grid.js';
@@ -30,6 +31,7 @@ import type { ContextMenuHost, MenuTarget } from '@utils/context-menu-controller
 import { FavoritesController } from '@store/controllers/favorites-controller';
 import { ViewLifecycleMixin } from '@utils/view-lifecycle';
 import { RovingGridController } from '@utils/roving-grid';
+import { prefetchImageWindow } from '@utils/image-prefetch';
 
 import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import '@awesome.me/webawesome/dist/components/popup/popup.js';
@@ -582,6 +584,26 @@ export class ArtistsView
      * Scroll position persistence
      * ================================================================ */
 
+    /**
+     * Warm the avatars just past the rendered range (#65).
+     *
+     * `rangeChanged` is the rendered range and `visibilityChanged` is
+     * what is on screen; the virtualizer has already drawn about
+     * 1000px past the latter, so that is the wrong anchor to measure a
+     * prefetch window from. It is deliberately outside the
+     * `restoringScroll` guard below: a restored scroll lands in the
+     * middle of the grid, which is exactly when nothing around it is
+     * cached.
+     */
+    private onRangeChanged = (e: RangeChangedEvent) => {
+        prefetchImageWindow(
+            this.cachedGridEntries,
+            e.first,
+            e.last,
+            (entry) => this.artistAvatarURL(entry.artist),
+        );
+    };
+
     /**
      * Save the first visible item index on scroll.
      */
@@ -1145,7 +1167,16 @@ export class ArtistsView
      * Helpers
      * ================================================================ */
 
-    private renderArtistAvatar(artist: library.Artist) {
+    /**
+     * The image this artist's card will draw, or `''` for the initial
+     * placeholder.
+     *
+     * Split out of `renderArtistAvatar` so the prefetch (#65) asks for
+     * exactly what the card is going to ask for — a second copy of the
+     * tier ladder would be a second thing to keep in step, and warming
+     * the wrong tier is a download that buys nothing.
+     */
+    private artistAvatarURL(artist: library.Artist): string {
         const needed = (this.imageSize ?? 176) * window.devicePixelRatio;
         let imageURL = '';
 
@@ -1172,6 +1203,12 @@ export class ArtistsView
             ) ?? '';
         }
 
+        return imageURL;
+    }
+
+    private renderArtistAvatar(artist: library.Artist) {
+        const imageURL = this.artistAvatarURL(artist);
+
         if (imageURL) {
             return html`<img
                 class="avatar-image"
@@ -1531,6 +1568,7 @@ export class ArtistsView
                     .keyFunction=${(entry: ArtistEntry) => entry.artist.ID}
                     .layout=${this.gridLayout}
                     @visibilityChanged=${this.onVisibilityChanged}
+                    @rangeChanged=${this.onRangeChanged}
                 ></lit-virtualizer>
             </div>
             ${this.renderContextMenu()}

frontend/src/components/bottom-nav/bottom-nav.ts

@@ -4,6 +4,7 @@ import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import '@awesome.me/webawesome/dist/components/drawer/drawer.js';
 import type WaDrawer from '@awesome.me/webawesome/dist/components/drawer/drawer.js';
 import { designTokens } from '../../styles/tokens.css';
+import { sheetScrollFade } from '../../styles/sheet-scroll.css';
 import '../sidebar/app-sidebar.js';
 import { nameDialog } from '@utils/name-dialog';
 import { ICON_PLAYLIST } from '@utils/icon-language';
@@ -167,6 +168,15 @@ export class BottomNav extends LitElement {
             overflow: hidden;
         }
 
+        /* And this list does not fit (#210): measured at 424x439 with
+           the seed's eight destinations, the body is scrollHeight 412
+           against clientHeight 373, and eleven items at 48px would be
+           528 -- the count is the user's since #25. So the sheet says
+           where the fold is, with styles/sheet-scroll.css's two layers
+           rather than a second answer to the question #207 settled for
+           the context sheet. The colour is the local half: the sidebar
+           paints --yj-bg-surface, so the cover does too, or the fade
+           draws the menus' grey across the bottom of this one. */
         wa-drawer::part(body) {
             padding: 0;
             /* A scroll that reaches the end of this list must not
@@ -177,6 +187,23 @@ export class BottomNav extends LitElement {
                on a gesture-navigation phone -- the same allowance the
                bar itself makes above. */
             padding-bottom: env(safe-area-inset-bottom, 0);
+            --yj-sheet-surface: var(--yj-bg-surface, #212529);
+            ${sheetScrollFade}
+        }
+
+        /* And the sheet paints that surface once. The sidebar's host
+           paints the same grey -- which in the shell is the sidebar's
+           own background and here is a second, opaque copy of the
+           sheet's, drawn *over* the body's layers. So the fade was
+           painted and then covered: measured at 424x439 before this
+           rule, the last 32px read a flat 52,58,64 with 39px still
+           below. menu-surface meets the same requirement from the
+           other side, where .context-menu-panel[data-sheet] is
+           background-color: transparent; nothing changes visually
+           here, because the colour underneath is the one being
+           removed. */
+        app-sidebar {
+            background-color: transparent;
         }
 
         /* A sheet is dragged at with a thumb, so it says where its top

frontend/src/components/cover-grid/cover-grid.ts

@@ -8,6 +8,7 @@ import {
 import '@lit-labs/virtualizer';
 import type {
     LitVirtualizer,
+    RangeChangedEvent,
     VisibilityChangedEvent,
 } from '@lit-labs/virtualizer';
 import { grid } from '@lit-labs/virtualizer/layouts/grid.js';
@@ -30,6 +31,7 @@ import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import '@components/playlist-picker/playlist-picker.js';
 import { loadTrackDetails } from '@utils/lazy-track-details.js';
 import { tracksByFilePath, tracksForPaths } from '@utils/track-index.js';
+import { prefetchImageWindow } from '@utils/image-prefetch.js';
 import type { TrackDetails } from '@components/track-details/track-details.js';
 import type { CoverArtUrls } from '@components/track-details/track-details.js';
 import { AlbumSelectionManager } from './album-selection.js';
@@ -910,6 +912,31 @@ export class CoverGrid
         );
     };
 
+    /**
+     * Warm the covers just past the rendered range (#65).
+     *
+     * `rangeChanged` rather than `visibilityChanged`, because the two
+     * report different ranges and only one of them is the right
+     * anchor: visibility is what is on screen, and the virtualizer has
+     * already rendered about 1000px past that. Measured from the
+     * visible range this would spend most of its window on cards that
+     * already exist and have already asked for their own art.
+     *
+     * The entry lists are memoized, so asking for one here costs a
+     * reference compare.
+     */
+    private onRangeChanged = (e: RangeChangedEvent) => {
+        const entries = this.splitMode
+            ? this.getBeforeEntries()
+            : this.buildGridEntries();
+
+        prefetchImageWindow(entries, e.first, e.last, (entry) =>
+            entry.album.CoverArtPath
+                ? this.getCoverUrl(entry.album)
+                : '',
+        );
+    };
+
     /* ====================================================================
      * Virtualizer items
      * ==================================================================== */
@@ -2003,6 +2030,7 @@ export class CoverGrid
                 @keydown=${this.onGridAlbumKeydown}
                 @contextmenu=${this.onGridAlbumContextMenu}
                 @visibilityChanged=${this.onVisibilityChanged}
+                @rangeChanged=${this.onRangeChanged}
             ></lit-virtualizer>
         `;
     }
@@ -2037,6 +2065,7 @@ export class CoverGrid
                 @keydown=${this.onGridAlbumKeydown}
                 @contextmenu=${this.onGridAlbumContextMenu}
                 @visibilityChanged=${this.onVisibilityChanged}
+                @rangeChanged=${this.onRangeChanged}
             ></lit-virtualizer>
 
             <album-dropdown

frontend/src/components/menu-surface/menu-surface.ts

@@ -65,6 +65,7 @@ import '@awesome.me/webawesome/dist/components/popup/popup.js';
 import '@awesome.me/webawesome/dist/components/dialog/dialog.js';
 import type WaPopup from '@awesome.me/webawesome/dist/components/popup/popup.js';
 
+import { sheetScrollFade } from '../../styles/sheet-scroll.css';
 import { PHONE_QUERY } from '@utils/breakpoints';
 import { nameDialogsIn } from '@utils/name-dialog';
 
@@ -164,46 +165,17 @@ export class MenuSurface extends LitElement {
            and worse when the cut lands on a row boundary, where the
            sheet ends in a clean edge that reads as the end of the list.
 
-           Two layers, and the *order* is what asks the question: a
-           shadow pinned to the bottom of the box (attachment scroll),
-           and over it a cover of the sheet's own colour painted at the
-           end of the *content* (attachment local), which therefore
-           scrolls up over the shadow and hides it exactly when there is
-           nothing more to see. So the affordance is absent on a menu
-           that fits, present the moment one does not, and gone again at
-           the end of the list -- with no scroll listener, no
-           measurement, and nothing reaching into wa-dialog's shadow
-           root for the scroller. background-attachment is Chrome 4;
-           the reference device is Chrome 113.
-
-           **The curve is steep because the rows under it stay live.**
-           A scrim over a menu item is that item's text surface, and
-           this app's rule is that text clears 4.5:1 on every surface it
-           can sit on -- which the light ramp, whose bgElevated is
-           #e9ecef, is what makes non-theoretical. A row is 48px with
-           its label centred, so 32px of scrim that is already down to
-           a quarter strength at 14px reaches y-centre at about 0.06 and
-           spends its weight on the strip below the last legible label.
-           Measured on the dark ramp at x=300, flat 52,58,64 throughout
-           before: 50,56,62 at y=330, 33,37,40 at y=350 and 22,24,27 at
-           the bottom edge, and flat again at the end of the list. The
-           light ramp puts 9.9:1 on the last label. */
+           The two layers that say it live in styles/sheet-scroll.css
+           (#210), because the phone has a second sheet -- bottom-nav's
+           "More" -- which overflows for the same reason and must not
+           arrive at its own answer for what a fold looks like. What is
+           local to this sheet is the colour the cover is painted in:
+           the menus' elevated grey, handed over as --yj-sheet-surface
+           on the same box. */
         wa-dialog::part(body) {
             padding: 0;
-            overflow-y: auto;
-            background:
-                linear-gradient(
-                        var(--yj-bg-elevated, #343a40),
-                        var(--yj-bg-elevated, #343a40)
-                    )
-                    bottom / 100% 32px no-repeat local,
-                linear-gradient(
-                        to top,
-                        rgba(0, 0, 0, 0.6) 0%,
-                        rgba(0, 0, 0, 0.25) 45%,
-                        rgba(0, 0, 0, 0) 100%
-                    )
-                    bottom / 100% 32px no-repeat scroll;
+            --yj-sheet-surface: var(--yj-bg-elevated, #343a40);
+            ${sheetScrollFade}
         }
 
         /* A sheet is dragged at with a thumb, so it says where its top

frontend/src/styles/sheet-scroll.css.ts

@@ -0,0 +1,68 @@
+import { css } from 'lit';
+
+/**
+ * A bottom sheet whose body scrolls says so, in one rule both sheets
+ * read.
+ *
+ * The app has two sheets — `menu-surface`'s context menu (#60) and
+ * `bottom-nav`'s "More" navigation (#71) — and both are capped at 85vh,
+ * because a surface covering the whole screen is a page rather than a
+ * sheet. So both overflow, and both used to overflow *silently*: the
+ * menu at 424x439 with eight items ending at y=470 (#207), the nav
+ * sheet at the same viewport with `scrollHeight` 412 against
+ * `clientHeight` 373 (#210). Where the cut lands on a row boundary the
+ * sheet ends in a clean edge that reads as the end of the list.
+ *
+ * The mechanism is #207's and is unchanged by being shared: two
+ * background layers on the scrolling box, whose *attachments* are the
+ * conditionality. A cover of the sheet's own colour is painted at the
+ * end of the *content* (`local`) over a shadow pinned to the box
+ * (`scroll`), so the cover scrolls up over the shadow exactly when
+ * there is nothing more to see. The fade is therefore absent on a sheet
+ * that fits, present the moment one does not, and gone again at the end
+ * of the list — with no scroll listener, no measurement and nothing
+ * reaching into another component's shadow root for the scroller.
+ * `background-attachment` is Chrome 4; the reference device is
+ * Chrome 113.
+ *
+ * Three things about it are load-bearing.
+ *
+ * **The cover takes the sheet's own colour, from a custom property.**
+ * The two sheets are different greys — the nav sheet paints
+ * `--yj-bg-surface`, because it holds the sidebar and two greys in one
+ * sheet is a seam across the middle of it, while the context sheet
+ * paints the menus' `--yj-bg-elevated`. A shared rule that hard-coded
+ * either would put that seam back on the other one, so the host sets
+ * `--yj-sheet-surface` on the same box and this reads it.
+ *
+ * **The curve is steep because the rows under it stay live.** A scrim
+ * over a menu item is that item's text surface, and this app's rule is
+ * that text clears 4.5:1 on every surface it can sit on — which the
+ * light ramp, whose `bgElevated` is `#e9ecef`, makes non-theoretical. A
+ * row is 48px with its label centred, so 32px of scrim already down to
+ * a quarter strength at 14px spends its weight on the strip below the
+ * last legible label: measured at 9.9:1 on that label on the light ramp,
+ * against 5.0:1 for a linear 48px draft at 0.8. The dark-ramp pixel
+ * table is in `.planning/NOTES.md` (2026-08-23).
+ *
+ * **The box is declared a scroller here too.** `overflow-y: auto` is
+ * part of the same statement rather than left to each host: a fade over
+ * a box that is not the scroller is a fade that never moves, and the
+ * component tier asserts the pair together for that reason.
+ */
+export const sheetScrollFade = css`
+    overflow-y: auto;
+    background:
+        linear-gradient(
+                var(--yj-sheet-surface, #343a40),
+                var(--yj-sheet-surface, #343a40)
+            )
+            bottom / 100% 32px no-repeat local,
+        linear-gradient(
+                to top,
+                rgba(0, 0, 0, 0.6) 0%,
+                rgba(0, 0, 0, 0.25) 45%,
+                rgba(0, 0, 0, 0) 100%
+            )
+            bottom / 100% 32px no-repeat scroll;
+`;

frontend/src/utils/image-prefetch.ts

@@ -0,0 +1,156 @@
+/**
+ * Warm the browser's image cache for the cards a scroll is about to
+ * reach.
+ *
+ * #65: album art pops in while scrolling. The rule this app already
+ * follows is that a row image is `loading="lazy" decoding="async"` and
+ * draws the smallest adequate tier, and both halves are in place —
+ * `cover-grid.getCoverUrl()` and `artists-view`'s avatar both pick
+ * `_sm`/`_md`/`_lg` from the card size and the device pixel ratio. What
+ * is left is *when* the fetch starts: the grids are virtualized, so the
+ * `<img>` does not exist at all until the virtualizer decides to render
+ * its card, and only then can the browser ask for anything.
+ *
+ * The issue's Direction asks for a larger overscan, and that is not
+ * available: `@lit-labs/virtualizer`'s `_overhang` is a hard-coded
+ * 1000px `protected` field on `BaseLayout` with no configuration
+ * surface, so raising it means monkey-patching a private. 1000px is
+ * about two screens on the reference device's 439px viewport, which is
+ * a fraction of a second at speed.
+ *
+ * So the request is issued ahead of the element instead. Cover art and
+ * artist images are plain URLs served by `coverart.Handler` /
+ * `explore`'s image handler under `Cache-Control: public,
+ * max-age=31536000, immutable` — the filenames are content hashes — so
+ * a prefetched image is a cache hit by the time the card is drawn, and
+ * a second pass over the same rows costs nothing at all.
+ *
+ * Three things about it are load-bearing.
+ *
+ * **This is not the `LRUMap` path the issue's Findings warn about.**
+ * That ceiling (`ARTIST_IMAGE_CACHE_LIMIT` and friends) bounds
+ * Explore's base64 data URLs, which are held in JS. A library cover is
+ * a URL, and what retains the bytes is the browser's own HTTP cache,
+ * which evicts on its own terms. What this module retains is the *set
+ * of URLs already asked for*, which is why that set has a cap and
+ * reports itself to `window.__yjCacheStats()` — the measurement the
+ * issue asks for.
+ *
+ * **A window is warmed on both sides of the rendered range.** The
+ * event carries no direction, and scrolling back up needs the same
+ * treatment; the rows behind are already in `requested` from the pass
+ * that rendered them, so the backward half issues nothing in the
+ * common case and is free.
+ *
+ * **An in-flight image is held.** `new Image().src = url` and drop it
+ * is the usual idiom and usually survives, but "usually" is an engine
+ * detail and the engine that matters here is a two-year-old WebView.
+ * The element is kept until it loads or fails, and no longer — nothing
+ * here holds a decoded bitmap on purpose.
+ */
+
+import { registerCacheProbe } from './cache-stats.js';
+import { LRUMap } from './lru-map.js';
+
+/**
+ * How many entries past each edge of the rendered range to warm.
+ *
+ * Entries rather than pixels, because that is what the event reports
+ * and what the caller has an array of. Twelve rows on the phone's
+ * two-column grid and four on a desktop's six, on top of the
+ * virtualizer's own 1000px — enough to cover a flick, and bounded so a
+ * fast scroll through 5 000 albums cannot ask for 5 000 covers.
+ */
+export const PREFETCH_AHEAD = 24;
+
+/** Ceiling on the record of what has already been asked for. */
+export const PREFETCH_MEMORY = 512;
+
+/** URLs already requested; the value is a placeholder, the key is the record. */
+const requested = new LRUMap<string, true>(PREFETCH_MEMORY);
+
+/** Images still loading, held so the request cannot be collected. */
+const inFlight = new Set<HTMLImageElement>();
+
+registerCacheProbe('imagePrefetch', () => {
+    let chars = 0;
+
+    for (const url of requested.keys()) chars += url.length;
+
+    return { entries: requested.size, chars, limit: PREFETCH_MEMORY };
+});
+
+/** Whether this URL has already been asked for. */
+export function imagePrefetched(url: string): boolean {
+    return requested.has(url);
+}
+
+/**
+ * Ask the browser for `url` unless it has already been asked for.
+ * Returns whether a request was issued.
+ */
+export function prefetchImage(url: string): boolean {
+    if (!url || requested.has(url)) return false;
+
+    requested.set(url, true);
+
+    const img = new Image();
+
+    inFlight.add(img);
+
+    const done = () => {
+        inFlight.delete(img);
+    };
+
+    img.addEventListener('load', done, { once: true });
+    img.addEventListener('error', done, { once: true });
+    img.decoding = 'async';
+    img.src = url;
+
+    return true;
+}
+
+/**
+ * Warm the images either side of a virtualizer's rendered range.
+ *
+ * `first`/`last` are the indices the `visibilityChanged` event
+ * reported; `urlOf` returns the image the card at that index will
+ * draw, or `''` where it draws a placeholder. Returns how many
+ * requests were issued, which is what a test can assert on and what
+ * makes "a second run does approximately nothing" checkable.
+ */
+export function prefetchImageWindow<T>(
+    items: readonly T[],
+    first: number,
+    last: number,
+    urlOf: (item: T) => string,
+    ahead: number = PREFETCH_AHEAD,
+): number {
+    if (items.length === 0 || first < 0 || last < first) return 0;
+
+    const from = Math.max(0, first - ahead);
+    const to = Math.min(items.length - 1, last + ahead);
+    let issued = 0;
+
+    // Forward first: it is the direction a scroll is usually going, so
+    // it is the half that has to win the race.
+    for (let i = last + 1; i <= to; i++) {
+        const item = items[i];
+
+        if (item !== undefined && prefetchImage(urlOf(item))) issued++;
+    }
+
+    for (let i = from; i < first; i++) {
+        const item = items[i];
+
+        if (item !== undefined && prefetchImage(urlOf(item))) issued++;
+    }
+
+    return issued;
+}
+
+/** Forget what has been asked for. For tests; the app never needs it. */
+export function resetImagePrefetch(): void {
+    requested.clear();
+    inFlight.clear();
+}

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/components/bottom-nav.test.ts

@@ -243,6 +243,62 @@ describe('bottom-nav', () => {
     expect(getComputedStyle(body).overscrollBehaviorY).toBe('contain');
   });
 
+  it('says where the fold is, in the sheet\'s own colour', async () => {
+    const el = await fixture<Nav>('bottom-nav');
+    const drawer = shadow<HTMLElement & { open: boolean }>(el, 'wa-drawer');
+
+    if (!drawer) throw new Error('no drawer');
+
+    const shown = once(drawer, 'wa-after-show');
+
+    shadow<HTMLButtonElement>(el, '[data-testid="tab-more"]')?.click();
+    await shown;
+
+    const body = drawer.shadowRoot?.querySelector('[part~="body"]');
+
+    if (!body) throw new Error('no body part to scroll');
+
+    const style = getComputedStyle(body);
+
+    // #210. This list does not fit the phone — measured at 424x439,
+    // `scrollHeight` 412 against `clientHeight` 373 with the seed's
+    // eight destinations — and said nothing about it, which where the
+    // cut lands on a row boundary reads as the end of the list.
+    //
+    // The mechanism is #207's and is asserted the same way: the pair of
+    // attachments *is* the feature. A cover of the sheet's own colour
+    // painted at the end of the content (`local`) over a shadow pinned
+    // to the box (`scroll`), so the fade is absent on a sheet that
+    // fits, present the moment one does not, and gone again at the end.
+    expect(
+      style.backgroundAttachment,
+      'the cover must be local and the shadow must not',
+    ).toBe('local, scroll');
+    expect(style.backgroundPosition).toBe('50% 100%, 50% 100%');
+    expect(style.backgroundSize).toBe('100% 32px, 100% 32px');
+
+    // And the colour is the local half of a shared rule: this sheet
+    // paints the sidebar's `--yj-bg-surface` (#212529) rather than the
+    // menus' elevated grey, or the fade draws the *other* sheet's
+    // colour across the bottom of this one — which is the seam a
+    // shared fragment would otherwise reintroduce.
+    expect(style.backgroundImage).toMatch(
+      /^linear-gradient\(rgb\(33, 37, 41\), rgb\(33, 37, 41\)\)/,
+    );
+
+    // And nothing paints over it. The sidebar's host carries the same
+    // grey, which inside the sheet is a second opaque copy of the
+    // surface drawn on top of these layers -- measured at 424x439 with
+    // the rule removed, the last 32px read a flat 52,58,64 with 39px
+    // still below, so the fade was painted and covered. That is
+    // `.context-menu-panel[data-sheet]`'s transparency, one sheet over.
+    const sidebar = shadow<HTMLElement>(el, 'app-sidebar');
+
+    if (!sidebar) throw new Error('no sidebar');
+
+    expect(getComputedStyle(sidebar).backgroundColor).toBe('rgba(0, 0, 0, 0)');
+  });
+
   it('gives the sheet the whole width, which the sidebar does not take', async () => {
     const el = await fixture<Nav>('bottom-nav');
 

frontend/test/components/menu-surface.test.ts

@@ -254,6 +254,15 @@ describe('menu-surface', () => {
       // Both sit at the bottom, or the cover hides nothing.
       expect(style.backgroundPosition).toBe('50% 100%, 50% 100%');
       expect(style.backgroundSize).toBe('100% 32px, 100% 32px');
+
+      // The layers are shared with `bottom-nav`'s sheet since #210, and
+      // the colour is what each host still says for itself: this one
+      // paints the menus' `--yj-bg-elevated` (#343a40). A shared rule
+      // that hard-coded one grey would draw a seam across the other
+      // sheet, which is why the fragment reads a custom property.
+      expect(style.backgroundImage).toMatch(
+        /^linear-gradient\(rgb\(52, 58, 64\), rgb\(52, 58, 64\)\)/,
+      );
     });
 
     /**

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);
+  });
+});