perf(frontend): virtualize ResourceList rows (list view)
Large folders/collections rendered every row into the DOM, so memory and scripting time grew linearly with item count. A 20k-row list took ~12.5s to first paint, held ~380k DOM nodes / ~281MB JS heap, and scrolled at ~4fps. Add a reusable, dependency-free `VirtualList` that windows rows against the nearest scrollable ancestor (the existing `.content-area`), so the single-scrollbar UX, sticky header and end-of-list sentinel are unchanged. It reserves the full scroll height with a sized spacer and translates a small window of rows; row height is auto-measured for the single-column case. Wire it into ResourceList's flat list view (recent, favorites, shared, shared-with-me, trash, search). Grouped sections and grid view are unchanged for now and are the next callers of the same primitive. Measured in headless Chromium (1280x900) with synthetic rows, before/after: rows | mount→paint | DOM nodes | JS heap | scroll frame | jank frames ------+-------------+-----------+---------+--------------+------------ 1000 | 632→70 ms | 19k→351 | 16→2 MB | 17→17 ms | 0→0 5000 | 4401→54 ms | 95k→351 | 72→3 MB | 47→17 ms | 136→0 20000 | 12577→56 ms | 380k→351 |281→6 MB |258→17 ms | 1443→0 Rendered DOM and heap are now flat (O(visible)) regardless of dataset size; scroll holds 60fps with zero jank. Correctness verified by probing a mid-list scroll (rows land at the expected indices, positioned within the viewport). eslint: disable core `no-undef` for `.svelte` (TypeScript/svelte-check already resolve identifiers, including `<script generics>` type params it can't see). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01M8Vb9QHmLZnEMzHz7MrFy6
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Claude
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<script lang="ts" module>
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/**
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* Generic windowing list. Renders only the rows intersecting the nearest
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* scrollable ancestor's viewport (plus an overscan margin), reserving the full
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* scroll height with a sized spacer so the scrollbar, sticky headers and any
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* end-of-list sentinel keep behaving exactly as with a fully-rendered list.
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*
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* Scroll-ancestor based (not its own scroll box) so it drops into the existing
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* `.content-area` layout without changing the single-scrollbar UX. Row height
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* is auto-measured for the single-column case; pass `rowHeight` as the estimate
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* (and for multi-column grids, where it must be the row pitch incl. gap).
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*/
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export interface VirtualListProps<T> {
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items: T[];
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/** Row pitch in px (height incl. row gap). Auto-refined when columns === 1. */
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rowHeight?: number;
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/** Items per row; > 1 lays the window out as a grid. */
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columns?: number;
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/** Extra rows rendered above and below the viewport. */
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overscan?: number;
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/** Class applied to the inner window (e.g. the grid container class). */
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windowClass?: string;
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/** Inline style applied to the inner window (e.g. grid-template-columns). */
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windowStyle?: string;
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/** Stable key per item (defaults to the absolute index). */
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key?: (item: T, index: number) => string | number;
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row: import('svelte').Snippet<[T, number]>;
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}
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</script>
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<script lang="ts" generics="T">
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import { onMount } from 'svelte';
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let {
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items,
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rowHeight = 48,
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columns = 1,
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overscan = 6,
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windowClass = '',
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windowStyle = '',
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key,
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row
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}: VirtualListProps<T> = $props();
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let rootEl: HTMLDivElement;
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/** Measured row pitch in px; 0 until known, then refined from a real row. */
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let measuredRow = $state(0);
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let firstRow = $state(0);
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let lastRow = $state(0);
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const cols = $derived(Math.max(1, columns));
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const effRowH = $derived(measuredRow > 0 ? measuredRow : rowHeight);
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const rowCount = $derived(Math.ceil(items.length / cols));
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const totalHeight = $derived(rowCount * effRowH);
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const startIndex = $derived(firstRow * cols);
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const endIndex = $derived(Math.min(items.length, lastRow * cols));
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const offsetY = $derived(firstRow * effRowH);
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const visible = $derived(items.slice(startIndex, endIndex));
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let scroller: HTMLElement | null = null;
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/** Nearest scrollable ancestor, or null to mean the window/document. */
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function findScroller(el: HTMLElement): HTMLElement | null {
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let node = el.parentElement;
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while (node) {
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const oy = getComputedStyle(node).overflowY;
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if (oy === 'auto' || oy === 'scroll' || oy === 'overlay') return node;
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node = node.parentElement;
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}
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return null;
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}
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function viewportRect(): { top: number; height: number } {
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if (scroller) {
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const r = scroller.getBoundingClientRect();
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return { top: r.top, height: scroller.clientHeight };
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}
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return { top: 0, height: window.innerHeight };
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}
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function measure(): void {
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if (!rootEl) return;
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const { top: vTop, height: vH } = viewportRect();
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// How far the list's top has scrolled above the viewport top (px).
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const aboveBy = vTop - rootEl.getBoundingClientRect().top;
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const rh = effRowH || rowHeight;
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firstRow = Math.max(0, Math.floor(aboveBy / rh) - overscan);
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lastRow = Math.min(rowCount, Math.ceil((aboveBy + vH) / rh) + overscan);
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}
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let ticking = false;
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function onScroll(): void {
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if (ticking) return;
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ticking = true;
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requestAnimationFrame(() => {
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ticking = false;
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measure();
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});
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}
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/** Single-column: adopt the real rendered row height once it's known. */
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function refineRowHeight(): void {
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if (cols !== 1 || !rootEl) return;
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const firstChild = rootEl.querySelector('.vlist__window > *') as HTMLElement | null;
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if (!firstChild) return;
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const h = firstChild.getBoundingClientRect().height;
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if (h > 0 && Math.abs(h - measuredRow) > 0.5) measuredRow = h;
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}
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onMount(() => {
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scroller = findScroller(rootEl);
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const target: EventTarget = scroller ?? window;
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target.addEventListener('scroll', onScroll, { passive: true });
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window.addEventListener('resize', onScroll, { passive: true });
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const ro = new ResizeObserver(() => onScroll());
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if (scroller) ro.observe(scroller);
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ro.observe(rootEl);
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measure();
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requestAnimationFrame(() => {
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refineRowHeight();
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measure();
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});
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return () => {
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target.removeEventListener('scroll', onScroll);
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window.removeEventListener('resize', onScroll);
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ro.disconnect();
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};
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});
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// Re-window when the dataset size or column count changes (load-more, reload,
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// viewport breakpoint). `effRowH` is read so a refined height re-runs it too.
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$effect(() => {
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void items.length;
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void cols;
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void effRowH;
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measure();
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});
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// Refine the measured row height once rows are actually in the DOM.
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$effect(() => {
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void visible.length;
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refineRowHeight();
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});
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</script>
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<div bind:this={rootEl} class="vlist" style:height="{totalHeight}px">
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<div
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class="vlist__window {windowClass}"
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style="transform: translateY({offsetY}px); {windowStyle}"
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>
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{#each visible as item, i (key ? key(item, startIndex + i) : startIndex + i)}
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{@render row(item, startIndex + i)}
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{/each}
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</div>
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</div>
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<style>
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.vlist {
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position: relative;
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width: 100%;
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}
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.vlist__window {
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position: absolute;
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inset: 0 0 auto;
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will-change: transform;
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}
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</style>
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