Optimize chat composer context updates

[cursor]

What Changed

• Added a stable context-window selector for chat threads so activity-only updates reuse the previous context-window snapshot.
• Updated ChatComposer to subscribe to that selector directly and stopped passing the mutable full thread object / activities array through the memoized composer prop boundary.
• Added selector tests covering reuse on unrelated activity appends and refresh on real context-window updates.

Why

react-doctor flagged ChatView.tsx / ChatComposer.tsx as React hot spots with state/effect and rerender risks. A manual pass found that streaming/thread activity updates changed activeThread.activities and the full activeThread object, causing the composer subtree to rerender even when the context-window meter did not change. This keeps the high-frequency timeline updates from invalidating the composer unless composer-relevant data changes.

UI Changes

React Scan recordings captured against the same seeded chat route and activity-update scenario:

• Before: react_scan_before_context_activity.webm — composer flashes during repeated activity-only updates.
• After: react_scan_after_context_activity_v2.webm — thread area still updates, but the composer no longer flashes during the activity-only update phase.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Validation:

• corepack pnpm --filter @t3tools/web test -- src/storeSelectors.test.ts (web unit project: 1198 passed)
• corepack pnpm exec vp check (run with pre-existing uncommitted package.json edit temporarily stashed)
• corepack pnpm exec vp run typecheck

  

Note

Optimize ChatComposer context updates by deriving context window snapshot from the store

• ChatComposer now receives only activeThreadSessionProviderInstanceId instead of the full Thread object and its activities, reducing unnecessary re-renders when unrelated thread data changes.
• Adds createContextWindowSnapshotSelectorByRef in storeSelectors.ts, a memoized selector that computes the context window snapshot from the global store and returns a stable reference when unrelated activities are appended or derived values are unchanged.
• Adds contextWindowSnapshotsEqual in contextWindow.ts to perform field-level equality checks on snapshots, preventing reference churn when values are identical.
• Behavioral Change: terminal context insertion now requires activeThreadId rather than the full activeThread object.

^{Macroscope summarized 487295d.}

[github-actions]

🚀 Expo continuous deployment is ready!

• Project → t3-code
• Platforms → android, ios
• Scheme → t3code-preview

	🤖 Android	🍎 iOS
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