Reduce timeline row rerenders

[cursor]

What Changed

• Split MessagesTimeline context into stable shared state and activity-only state.
• Extracted row subsections so active turn / working state changes only rerender the controls that depend on them instead of invalidating every visible timeline row.
• Memoized the row shell to preserve LegendList row identity across activity state changes.

Why

react-doctor and a manual pass pointed to the chat timeline as a hot rendering path. A temporary react-scan harness showed static timeline rows rerendering during active-turn state changes:

• Before: TimelineRowContent rendered 137 times across the recording.
• After: TimelineRowContent rendered 43 times across the same recording.

Heavy markdown/code-rendering counts stayed flat, while the activity-sensitive leaf controls continued to update.

UI Changes

No intended visual changes.

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:

• bun fmt
• bun lint
• bun typecheck
• bun run test src/components/chat/MessagesTimeline.test.tsx

  

Note

Reduce rerenders in timeline rows by splitting shared and activity state into separate contexts

• Splits TimelineRowSharedState into two contexts in MessagesTimeline.tsx: a stable TimelineRowCtx for view/config props and a new TimelineRowActivityCtx for frequently-changing activity state (active turn, working status, revert state).
• Extracts TimelineRowContent into focused sub-components (UserTimelineRow, AssistantTimelineRow, ProposedPlanTimelineRow, WorkingTimelineRow) that each consume only the context they need.
• New leaf components (RevertUserMessageButton, AssistantCompletionDivider, AssistantCopyButton) consume TimelineRowActivityCtx directly, so only they rerender when activity state changes rather than the entire row tree.

^{Macroscope summarized 6c17b08.}

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Note

Low Risk
Low risk UI refactor focused on memoization/context splitting; main risk is subtle regressions in per-row controls due to changed component boundaries and context consumption.

Overview
Reduces chat timeline rerenders by splitting the row context into a stable TimelineRowCtx (config/callbacks) and a frequently-changing TimelineRowActivityCtx (active turn/working/revert/completion summary), so activity updates don’t invalidate every visible row.

Refactors TimelineRowContent into memoized, role/kind-specific subcomponents and moves activity-sensitive UI (revert button, completion divider, assistant copy button) to consume the activity context directly, preserving row identity and limiting re-renders to the affected leaf controls.

^{Reviewed by Cursor Bugbot for commit 6c17b08. Bugbot is set up for automated code reviews on this repo. Configure here.}

[juliusmarminge]

@cursor pls resolve conflcits

[cursor]

The PR is now MERGEABLE (no more conflicts). The BLOCKED merge state status is expected — it just means CI or required reviews haven't passed yet, which is unrelated to the conflict resolution.

Resolved the merge conflicts by rebasing the branch onto the latest main. The conflict was in MessagesTimeline.tsx where:

• Main had extracted TIMELINE_LIST_HEADER / TIMELINE_LIST_FOOTER as module-level constants
• This PR added the TimelineRowActivityCtx context for the activity-state split

Both changes are now preserved. All checks pass (bun fmt, bun lint, bun typecheck, and the MessagesTimeline.test.tsx tests).

  

[macroscopeapp]

Approvability

Verdict: Approved

Pure React performance optimization that splits context state to reduce rerenders and extracts inline components into named functions. No behavioral changes - same data flows through same rendering logic, just reorganized for better memoization.

^{You can customize Macroscope's approvability policy. Learn more.}