fix(mobile): Stabilize thread feed layout and Markdown rendering

[juliusmarminge]

Extracted from #3910.

Fixes native selectable Markdown width and stale-content behavior, keeps screenshots interactive, and stabilizes floating-composer spacing, short-content positioning, and hydration layout in the thread feed. Includes the required LegendList patch and regenerated lockfile.

Validation inherited from #3910: vp check and mobile native lint passed before the split. The full typecheck is currently blocked in this workspace by unavailable mobile dependencies (expo-blur, expo-quick-actions, and @tabler/icons-react-native).

Note

Stabilize thread feed scroll layout and fix Markdown rendering in iOS

• Fixes thread feed scroll position when content is shorter than the viewport by anchoring content under translucent headers and scrolling to end when content grows to fill the viewport.
• Adds a fillWidth prop to SelectableMarkdownText so assistant markdown messages stretch to fill the full row width.
• Refactors T3MarkdownTextShadowNode to derive content via buildContent and gate state updates with equality checks, preventing redundant re-renders.
• Patches @legendapp/list to improve end-scroll maintenance, initial scroll readiness, and keyboard-aware inset handling near translucent headers.
• Adds measured composer overlay height tracking in ThreadDetailScreen to produce a stable resting inset above the composer.
• Behavioral Change: maintainScrollAtEnd is disabled while content underflows the viewport; scroll position is instead managed by a requestAnimationFrame loop.

^{Macroscope summarized 775d0b1.}

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Note

Medium Risk
Large behavioral changes in patched list scrolling and native shadow-node state publishing affect core chat UX on iOS; no auth or data-path changes, but regressions in scroll position or markdown rendering would be user-visible.

Overview
Fixes thread feed resting scroll, short-content positioning, and native selectable markdown width/stale text, with a refreshed @legendapp/list patch.

Native markdown (T3MarkdownText) stops caching measure-time attributed string in mutable members. buildContent() rebuilds from children on each measure/layout; layout() calls updateStateIfNeeded with equality on paragraph/attachment ranges so Yoga clones that skip measure no longer publish stale or empty text.

SelectableMarkdownText gains optional fillWidth (stretch in shrink-to-fit bubbles); thread assistant messages enable it.

Thread detail / feed: Measured floating composer height drives contentInsetEndEstimate and a 24px gap via useKeyboardChatComposerInset’s new heightAdjustment. ThreadFeed detects content shorter than the viewport, scrolls to the header inset, pauses maintainScrollAtEnd while underflowing, keeps the list mounted during loading hydration with a spinner overlay, and tightens list remount rules when the first message arrives.

LegendList patch adds contentInsetStartAdjustment, safe-area compensation for anchored end blank space, inset-end settle/reveal holds, corrected maintainScrollAtEnd threshold (raw scroll end vs inset-adjusted end), and related clamp/initial-scroll fixes for transparent-header iOS layouts. Lockfile patch hash updated.

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Approvability

Verdict: Needs human review

This bug fix touches native iOS C++ layout code and includes substantial patches to the @legendapp/list third-party library affecting scroll behavior calculations. While the author is the primary maintainer of these files, native code changes and third-party patches to rendering infrastructure warrant human review.

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