Tiles view: previews are fetched at 1000x1000 and never released

[JammingBen]

Part of the file list scroll performance work for #3159.

Current situation

ImageDimension.Tile is [1000, 1000] (packages/web-pkg/src/constants.ts). Each tile thumbnail is a ~816 KB response, decoded into roughly 4 MB of bitmap, and displayed in a 316x178 px box.

On top of that:

• previewService.privatePreviewBlob calls window.URL.createObjectURL() but nothing ever revokes those URLs, so no preview is ever freed
• itemVisible queues a preview for every item the user scrolls past, and nothing cancels queued work for items that have left the viewport
• cacheService.filePreview has a capacity of 250 for a 500-item page, so the second half evicts the first and previews are re-fetched when scrolling back
• Cache.evict() walks the whole map on every get and set
• useLoadPreview creates its own PQueue per caller, so concurrency is not bounded globally

Measured on a 500-image folder (items per page 500):

Scenario	Engine	Median frame	Tiles filled in 4 s
Tiles, baseline	WebKit	80 ms	108
Tiles, preview requests blocked	WebKit	32 ms	348
Tiles, baseline	Chromium	18 ms	448
Tiles, preview requests blocked	Chromium	10 ms	500

Sampled response: processor=fit&x=1000&y=1000, 836,221 bytes, 37-41 ms each. 500 tiles is roughly 400 MB of image data.

The table view is much less affected (WebKit 47 ms → 39 ms) because table thumbnails are 36x36.

Why this is bad

Preview loading is the single biggest cost in the tiles view on Safari - roughly 60% of the frame time. The unbounded memory growth also degrades long sessions, and re-fetching on scroll-back wastes bandwidth and puts avoidable load on the thumbnails service.

Proposed solution

1. Derive the tile preview size from the rendered tile size multiplied by devicePixelRatio and capped - roughly 640x640 instead of 1000x1000. Leave ImageDimension.Preview alone for the preview app and SpaceHeader. perf: dynamic tile preview dimensions #3192
2. Revoke object URLs when a preview cache entry is evicted or replaced, and when the resource list is cleared. perf: preview cache optimizations #3194
3. Raise the preview cache capacity to at least the page size so scrolling back does not re-fetch, and replace the O(n) evict() walk with lazy expiry plus proper LRU ordering. perf: preview cache optimizations #3194
4. Make the preview queue viewport-aware: drop or deprioritize queued items that are no longer visible, and share one queue across the list instead of one per useLoadPreview caller. perf: drop queued previews leaving the viewport #3198
5. Set decoding="async" on OcImages for loading thumbnails (in: ResourceTile, ResourceListItem and Projects). perf: decode thumbnails asynchronously #3204

Verification

• Safari tiles median frame time approaching the 32 ms measured with previews blocked
• transferred preview bytes for a 500-item folder down by roughly 4x
• JS heap flat after scrolling down and back up