[incognitojam]

The mobile message image grid from #1175 had no tests, so it had no fork feature ledger entry, and upstream intake batch 49 (pingdotgg#23135) had to re-apply it by hand when it rebuilt ThreadFeed's message body.

This moves the grid's decisions out of ThreadFeed.tsx into a pure helper, messageImageGrid.ts, and tests them:

• which layout a message gets: nothing, the large single image, or a grid of at most four tiles with a "+N" count of the rest;
• the tile size: two columns around the gap, capped at 240px, never negative;
• which user attachments go to the grid and which to the file list: images (including files with an image MIME type) always go to the grid, even when the text references them; other files the text references are left out of the list because they show as chips.

ThreadFeed renders the same views from the helper's result, so the grid behaves as before. The new mobile-message-image-grid ledger entry records the invariants, paths, and tests. It is marked review-needed with upstream status partial: upstream pingdotgg#27373, which the fork has not taken yet, moves user message images into a collage that shows every image with no tile limit or overflow count, while assistant images still stack. Intake of that PR needs to decide which layout the fork keeps.

Validation: compared the batch 49 version of ThreadFeed.tsx with the #1175 diff: the grid component, tile size, pending/sent/assistant call sites, and the grid/file-list split are all unchanged apart from the new body wrapper, whose px-3.5 padding matches the tile width calculation. The new tests cover each case in the list above.

Also checked on an Android emulator (Pixel 7, Android 17) against a dev server with synthetic fixture threads:

• A user message with seven images and a PDF shows four square tiles with "+3" on the fourth, then the PDF in the file list, then the text. The assistant reply with two images shows a two-tile grid across the content width.
• Tapping the "+3" tile opens the gallery at screen-4.png, and swiping goes on to screen-5.png, one of the hidden images.
• A message whose text references three images shows them in the grid, with the file names still shown as chips in the text.

I didn't capture a message with a single referenced image on the emulator; the tests cover that it keeps the single layout.

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Written by an agent (Claude Code, claude-opus-5-5).

[github-actions]

Upstream divergence

Both sides are compared with upstream at the in-order tip aaf4a901c4. Excludes .repos/ and pnpm-lock.yaml.

Measure	Base	This PR	Change
Upstream files the fork changes	1,778	1,778	0
Lines changed in them	106,547	106,556	+9
Upstream files the fork removes	45	45	0
Fork-only files	773	775	+2
Fork-only lines	135,451	135,657	+206

Upstream files whose difference changes (1)

File	Change	Now differs by
apps/mobile/src/features/threads/ThreadFeed.tsx	+9	454 lines