refactor(app): extract session turn changes

[Astro-Han]

Topology Update (2026-05-16)

Final base: dev after #672 was squash merged.
Dependency status: true stack dependency on the message-timeline owner context from #670/#671/#672; this PR has been restacked to latest dev.
Review order: review and merge after #672, before #675.
Latest head after restack and review follow-up: 8981175f0.

Summary

Extracted MessageTimeline turn-change ownership into packages/app/src/pages/session/session-turn-changes.tsx and added focused tests for the extracted pure helpers.

Why

This continues the #601 message-flow architecture stack after #672. MessageTimeline was still carrying turn-change fetch/cache/retry/dialog state inside the render component, which made the launch-path timeline harder for another agent to read or safely split before virtualization work.

Related Issue

• [Task] UI rewrite v2 Area A: message flow modular rollout #601
• Parent: [Feature] UI rewrite v2: modular, perf-gated rollout #599
• Stack child: refactor(app): remove orphaned timeline header code #675

Human Review Status

Fresh-eyes review clean. The follow-up review after 8981175f0 found no P0-P3 issues and confirmed the restack diff only contains this PR's turn-change extraction changes.

Review Focus

• Confirm the PR only moves the turn-change owner out of MessageTimeline and keeps SessionTurn props/behavior unchanged.
• Check the new session-turn-changes.tsx boundary, especially conflict dialog retry flow and blocked toast descriptions.
• Confirm the restack diff contains only this PR's turn-change extraction changes.

Risk Notes

Behavior is intended unchanged. Main risk is wiring regression in undo/redo turn-change fetch or conflict-confirm retry because that logic moved modules. No platform, dependency, persistence, or migration change.

Architecture Boundary

Owner lane: #601 message flow.

Base/depends on: dev. #667, #669, #670, #671, and #672 are merged into dev; #675 depends on this PR.

Touched files:

• packages/app/src/pages/session/message-timeline.tsx
• packages/app/src/pages/session/session-turn-changes.tsx
• packages/app/src/pages/session/session-turn-changes.test.ts

Architecture effect:

• owner extracted
• verification added
• message-timeline.tsx remains large but sheds turn-change ownership in this slice

Review Follow-up

• Gemini blocked-copy separator threads fixed and resolved. 730de8916 added the missing separator in the focused fake translator path.
• Fresh-eyes P2 locale separator finding fixed in 8981175f0; real locales continue to own their separator prefix, avoiding duplicate punctuation in English and Chinese.

How To Verify

Focused tests: bun test --preload ./happydom.ts src/pages/session/session-turn-changes.test.ts src/pages/session/session-message-comments.test.ts -> 8 pass, 0 fail
bun run typecheck -> 8 successful, 8 total
Targeted #600 timeline perf: bun script/e2e-local.ts -- e2e/perf/perf-probe.spec.ts --grep "long-session-input-lag|session-timeline-recompute" -> 1 passed, 1 skipped
git diff --check origin/dev...HEAD && git diff --check -> pass
GitHub required checks on 8981175f0: all passed, including perf-probe-baseline
Review threads: all resolved

Screenshots or Recordings

Not included. This PR is a behavior-preserving controller extraction with no intended visible UI or copy change, so Electron manual visual verification was not run.

Checklist

• Human review status is stated above as pending, approved, or not required
• I linked the related issue, or stated why there is no issue
• This PR has type, primary area, and priority labels, or I requested maintainer labeling
• I described the review focus and any meaningful risks
• I listed the relevant verification steps and the key result for each
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope
• I manually checked visible UI or copy changes when needed, with screenshots or recordings
• I considered macOS and Windows impact for platform, packaging, updater, signing, paths, shell, or permissions changes
• I called out docs, release notes, dependencies, permissions, credentials, deletion behavior, generated content, or local file changes when relevant
• I reviewed the final diff for unrelated changes and suspicious dependency changes
• I am targeting dev after the merged stack base, and my PR title and commit messages use Conventional Commits in English

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Run ID: 3107c955-53d5-4abd-a91e-fa9aeafe19a4

📥 Commits

Reviewing files that changed from the base of the PR and between 4dfea3f and 8981175.

📒 Files selected for processing (3)

• packages/app/src/pages/session/message-timeline.tsx
• packages/app/src/pages/session/session-turn-changes.test.ts
• packages/app/src/pages/session/session-turn-changes.tsx

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/message-timeline-turn-changes

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[gemini-code-assist]

Code Review

This pull request refactors the MessageTimeline component by extracting turn-change management logic—including fetching, caching, retries, and conflict dialogs—into a new createSessionTurnChanges controller. This change reduces the complexity of the timeline component, updates the frontend architecture manifest, and introduces unit tests for the extracted logic. Feedback identifies a missing separator in the blockedTurnChangeDescription function that results in poorly formatted file lists, and suggests updating the associated test expectations to reflect the corrected string concatenation.

[github-actions]

Perf delta summary

Comparator: pass

Profile / Scenario	interaction median	interaction worst	long task max	tbt	frame gap p95	frame gap max	jank count	cls	status
default / homepage-cold	40 -> 32 (-8)	56 -> 72 (+16)	88 -> 65 (-23)	38 -> 15 (-23)	33.4 -> 33.3 (-0.1)	133.4 -> 100 (-33.4)	3 -> 3 (0)	0 -> 0 (0)	pass
default / long-session-input-lag	40 -> 48 (+8)	48 -> 48 (0)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.7 (0)	16.8 -> 16.7 (-0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / session-streaming-long	48 -> 40 (-8)	72 -> 56 (-16)	0 -> 0 (0)	0 -> 0 (0)	16.8 -> 16.8 (0)	50 -> 16.8 (-33.2)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-call-expand	16 -> 24 (+8)	16 -> 24 (+8)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.7 (0)	16.7 -> 16.7 (0)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-default-open-heavy-bash	24 -> 24 (0)	32 -> 24 (-8)	65 -> 66 (+1)	15 -> 16 (+1)	33.4 -> 33.4 (0)	150 -> 166.6 (+16.6)	1 -> 2 (+1)	0 -> 0 (0)	pass
default / terminal-side-panel-open	48 -> 48 (0)	48 -> 56 (+8)	0 -> 0 (0)	0 -> 0 (0)	33.3 -> 33.4 (+0.1)	33.3 -> 33.4 (+0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / session-scroll-reading	24 -> 24 (0)	32 -> 32 (0)	0 -> 0 (0)	0 -> 0 (0)	16.8 -> 16.8 (0)	16.8 -> 16.8 (0)	0 -> 0 (0)	0.505 -> 0.505 (0)	warn: cls
low-end / session-scroll-reading-long	48 -> 48 (0)	48 -> 64 (+16)	72 -> 88 (+16)	35 -> 49 (+14)	16.8 -> 16.8 (0)	83.4 -> 83.3 (-0.1)	2 -> 3 (+1)	0.011 -> 0.011 (0)	pass
low-end / session-timeline-recompute	120 -> 120 (0)	128 -> 120 (-8)	105 -> 102 (-3)	160 -> 170 (+10)	83.4 -> 83.4 (0)	166.7 -> 183.3 (+16.6)	3 -> 3 (0)	0.081 -> 0.081 (0)	pass
low-end / concurrent-shimmer-extreme	0 -> 0 (0)	0 -> 0 (0)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.8 (+0.1)	16.8 -> 16.8 (0)	0 -> 0 (0)	0 -> 0 (0)	pass

[github-actions]

Suggested priority: P2 (includes user-path files (packages/app/src/pages/session/message-timeline.tsx, packages/app/src/pages/session/session-turn-changes.test.ts, packages/app/src/pages/session/session-turn-changes.tsx)).

P1/P0 are reserved for maintainer confirmation. Please relabel manually if this is a release blocker, security issue, data-loss risk, or updater/runtime failure.