Reduce chat timeline activity rerenders

[cursor]

What Changed

• Moved active-turn copy visibility and completion summary into the derived MessagesTimelineRow data.
• Narrowed the timeline activity context to only the revert button state that truly remains global.
• Added coverage proving only the active assistant turn is marked streaming for copy controls.
• Added React Scan recordings:
  • Before: artifacts/react-scan-before/settings-connections-before.webm
  • After: artifacts/react-scan-after/settings-connections-after.webm

Why

React Doctor highlighted the chat timeline as a large/hot component area. A manual pass found that TimelineRowActivityCtx carried active-turn and completion-summary state used by assistant row controls, which forced unrelated assistant copy buttons to observe context changes even when their row data was stable. Projecting those values onto row data lets computeStableMessagesTimelineRows preserve unaffected row identity and limits React work to rows whose rendered state can actually change.

UI Changes

No intentional visual changes. The committed React Scan before/after recordings show the app running with React Scan enabled during the scan pass.

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

Verification:

• bun fmt
• bun lint
• bun typecheck
• bun run test --filter=@t3tools/web -- MessagesTimeline.logic.test.ts

  

Note

Reduce chat timeline rerenders by moving streaming state into row derivation

• Moves completionSummary, activeTurnInProgress, and activeTurnId out of the shared TimelineRowActivityCtx context and into deriveMessagesTimelineRows, so each row carries its own assistantCopyStreaming and completionSummary fields.
• Only the assistant row whose turn matches the active in-progress turn gets assistantCopyStreaming=true, preventing all assistant copy buttons from re-rendering on every streaming update.
• Updates isRowUnchanged to include the two new fields, ensuring rows update correctly when these values change.

^{Macroscope summarized 91ae7e9.}

---

Note

Medium Risk
Medium risk because it changes how assistant streaming state and completion summaries are derived and rendered, which could subtly affect copy-button visibility and divider labeling during active turns.

Overview
Reduces chat timeline rerenders by moving active-turn copy streaming state and completion divider summary text out of TimelineRowActivityCtx and onto each derived MessagesTimelineRow.

deriveMessagesTimelineRows now accepts completionSummary, activeTurnInProgress, and activeTurnId, and computes per-row assistantCopyStreaming and completionSummary; AssistantCopyButton/AssistantCompletionDivider consume these row fields, and stable-row equality (isRowUnchanged) accounts for them. Adds a test ensuring only the active assistant turn is treated as streaming for copy controls.

^{Reviewed by Cursor Bugbot for commit 91ae7e9. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 5478fd5. Configure here.}

[macroscopeapp]

Approvability

Verdict: Approved

Performance refactor that moves computation from render-time context reads to row derivation time, reducing unnecessary React rerenders. The logic is equivalent and a test confirms correct behavior.

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