perf(desktop-gpui): avoid decoding offscreen clip thumbnails

[AksharP5]

Scrolling down a long Clips sidebar can leave visible thumbnails waiting behind decodes for cards you cannot see. Opening the sidebar also eventually decodes and keeps every thumbnail, even if you never scroll.

Request a thumbnail from the existing card-bounds probe only when the card intersects its current clip mask. Keep the existing decoder, cache keys, failure handling and two-job limit. Previously unseen cards load when they enter the viewport; revisiting a card reuses its cached image.

Closes #2706.

Verification

After loading the first viewport and jumping to the end, time until all visible thumbnails finish decoding:

Clips	Main	This PR
20	240 ms	180 ms
100	2.35 s	180 ms
500	13.05 s	180 ms

Seven alternating paired runs of the unchanged native decoder, with two worker threads on an i5-14400F. An optimized probe replays request order observed from native GPUI scrolling, using a synthetic 1080p H.264 recording and a 390 × 600 viewport. This measures decoder completion, excluding UI frame pacing and GPU presentation.

An idle 100-clip sidebar also retains 8 thumbnails instead of 100: 0.63 MiB instead of 7.91 MiB of thumbnail pixels.

The native GPUI replay covers 23 before/after scenarios, including dark/light themes, short and empty lists, partially visible rows, scrolling with jobs pending, duplicate keys, decode failure, resizing and cached return visits. Visible pixel hashes match and concurrency stays at two.

Native app cargo check, formatting and all 23 editor_clips::tests pass. Strict Clippy reports the same 27 pre-existing diagnostics on unchanged main and this patch under Rust 1.99, with no new findings.

This applies independently to current main. Related #2705 reduces card rendering work; this change avoids offscreen thumbnail decoding. Checked the current open PRs and issues for duplicate work.

Confidence Score: 5/5

The PR appears safe to merge; no actionable issues were found.

Summary

Requests clip thumbnails from each card’s bounds probe only when the card intersects the current clip mask.

• Clip cards request thumbnails only when they enter view.

_{Reviews (1) · Last reviewed commit: "perf(desktop-gpui): avoid decoding offsc..." · Reviewed by Greptile}

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