[rohan-patnaik]

Large stored image attachments can stall the timeline during decoding. Add decoding="async" to the regular user attachment image, including the existing HEIC wrapper. This keeps image URLs and layout intact and applies to web and the shared Electron UI.

Fixes #19922. Maintainer takeover of #19923; original implementation and synthetic reproduction by @rohan-patnaik. Original author commit retained; main merged without rebasing.

At 25b51d885d:

• 195 existing tests pass across MessagesTimeline.logic.test.ts, timelineScrollAnchoring.test.tsx, and ExpandedImagePreview.test.ts. Added tests: 0 before → 0 after; no attribute-only assertions.
• Web typecheck, scoped lint, formatting, and git diff --check pass.
• Fresh independent Claude Opus 5.5 and Codex GPT-6 Sol full-diff reviews: one round, no code findings.

The running app was measured in headless Linux Chromium at 1280 × 800 with 13 synthetic 4590 × 8160 stored JPEGs and fresh blob URLs for cold decoding. Three six-second unrecorded trials had maximum frame gaps of 2800/2917/2950 ms on the base versus 267/2567/1317 ms with async decoding. P95 was 17–33 ms on the base and 83–133 ms after. All 13 images loaded. These results support reducing the worst freeze, not uniformly smoother frames; async decoding can defer visible pixels. Raw frame intervals. Current uploads resize images; this fixture represents stored originals.

Before (base, settled images):

After (head, same viewport, fixture, and scroll position):

Before recording · After recording. Recording adds overhead and is separate from the measurements.

Not verified: macOS, Windows, iOS, Android, native Electron desktop, Safari, Firefox, or remote/relay/tunnel browsers. No provider turn was sent. Human review of the performance tradeoff remains required; unmerged.

Model: GPT-6.1 Sol (high), Codex harness in T3 Code. Tests: GPT-6 Astra (high); reviews: Claude Opus 5.5 and GPT-6 Sol.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1383e03 and ee6a3c0.

📒 Files selected for processing: 1

• apps/web/src/components/chat/MessagesTimeline.tsx

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📝 Walkthrough

The user-message image preview now requests asynchronous image decoding.

Changes: User-message image preview

Layer / File(s)	Summary
Set image decoding hint apps/web/src/components/chat/MessagesTimeline.tsx	The user-message image preview requests asynchronous decoding.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to ee6a3

No actionable merge-blocking risk remains from this change.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: enabling asynchronous decoding for timeline image attachments.
Description check		The description covers the problem, change, scope, linked issue and PR context, verification results, performance measurements, screenshots, recordings, limitations, and agent details. It is sufficien…
Linked Issues check		Issue #19922 requires asynchronous decoding for regular user attachment images while preserving image URLs, pixels, layout, and expansion behavior. The PR adds decoding="async" to the `AttachmentIma…
Out of Scope Changes check		The whole-PR diff contains one added decoding="async" attribute in apps/web/src/components/chat/MessagesTimeline.tsx. The change directly addresses issue #19922 and does not add unrelated behavior…

Full details: Description check

Explanation

The description covers the problem, change, scope, linked issue and PR context, verification results, performance measurements, screenshots, recordings, limitations, and agent details. It is sufficiently complete for the template.

Full details: Linked Issues check

Explanation

Issue #19922 requires asynchronous decoding for regular user attachment images while preserving image URLs, pixels, layout, and expansion behavior. The PR adds decoding="async" to the AttachmentImage instance in MessagesTimeline.tsx. AttachmentImage forwards this prop to its regular <img> and existing HeicImage wrapper, whose <img> also receives the prop. The diff does not change the source URL, layout classes, or expansion handler. The reported existing tests and validation support the change. No issue requirement mandates a new automated test.

Full details: Out of Scope Changes check

Explanation

The whole-PR diff contains one added decoding="async" attribute in apps/web/src/components/chat/MessagesTimeline.tsx. The change directly addresses issue #19922 and does not add unrelated behavior, refactoring, or files.

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[juliusmarminge]

Taking over this one as maintainer for composer-chat-01. Resuming the existing worktree, preserving the focused async decode hint and original author commits; I will verify it on current main before pushing.