fix(mobile): folding the Duo keeps the thread's end or the reader's line in place#23149

🟠 High

https://github.com/pingdotgg/t3code/blob/5b5e9062d3ecec8c4a26f94cc1cb8b701bc45122/apps/mobile/src/features/threads/ThreadFeed.tsx#L2488

A streaming render during the 160 ms no-momentum fallback cancels finishUserScroll, leaving userScrollSessionRef true and end-follow disabled; subsequent at-end scroll events cannot resume following until another gesture or send. clearUserScrollSettle is recreated on every render, so the effect cleanup at this line runs on each render; make this cleanup unmount-only or stabilize the callback.

This effect is unchanged from main; this PR only turned the reset effects into an effect event. ThreadFeed is compiled by React Compiler (mobile sets reactCompiler: true, and src/reactCompilerCoverage.test.ts checks ThreadFeed.tsx), which memoizes clearUserScrollSettle because it only touches refs. Its identity is stable, so the cleanup runs on unmount, not on each streaming render.

🟠 High patches/@legendapp__list@3.3.5.patch:402

Appending a row while sizeChangeAnchor returns an end target leaves the list at the old end, so the new row remains below the viewport. prepareMVCP disables the anchor on dataChanged passes, but this guard still treats the callback’s target as applied and skips doMaintainScrollAtEnd; only skip maintenance when that pass actually restored the anchor, or apply the anchor on data changes too.

Fixed in 2b3f7fbf39e. prepareMVCP now records whether a pass is a data change, and doMaintainScrollAtEnd skips end maintenance only after size-change passes, the ones the anchor places. A row added while a fold settles now gets end maintenance, the same as outside the fold window.

🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Honor explicit iOS MVCP opt-outs.

ListComponent currently passes native maintainVisibleContentPosition for every iOS list. This overrides maintainVisibleContentPosition={false} in the pull-request screens. When rows above the viewport remeasure, native MVCP can shift the offset before the screens’ explicit scroll restoration runs. Gate the forced iOS prop on sizeChangeAnchor; ThreadFeed supplies that anchor on iOS.

The forced iOS maintainVisibleContentPosition predates this PR: main's patch already has it, with the reason in the comment above it. Re-enabling native MVCP mid-drag can compare a stale anchor with the 1e7 ScrollAdjust sentinel and scroll the whole list out of view. Screens that pass maintainVisibleContentPosition={false} still turn off the JS restoration. This PR only adds the size-change anchor on top.

@juliusmarminge Thanks for the clarification. I withdraw this finding.

The forced native iOS MVCP prevents the stale-anchor/1e7 sentinel failure. maintainVisibleContentPosition={false} still disables JS restoration. Gating native MVCP on sizeChangeAnchor would remove that protection from other lists. No change is needed here.

✅ Review thread resolved.

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