refactor(ui): extract session turn changes panel

[Astro-Han]

Topology Update (2026-05-16)

Final base: dev.
Dependency status: independent flat PR and parent of the short SessionTurn stack. #677 is stacked on this PR because both slices modify packages/ui/src/components/session-turn.tsx, and reviewing the diff-summary extraction is cleaner after this panel extraction lands.
Review order: review and merge #676 before #677. This PR is independent from #667/#669 and from the message-timeline stack #670 -> #671 -> #672 -> #674 -> #675.
Latest head after review follow-up: d50f897bd.

Summary

Extracted the SessionTurn turn-change summary/action/file panel into packages/ui/src/components/session-turn-changes-panel.tsx and moved the turn-change action type into the existing session-turn-changes.ts contract helper.

Why

This continues the #601 message-flow architecture work. session-turn.tsx still mixed assistant rendering, legacy diff summary, and turn-change UI. This PR gives the turn-change panel a clear owner before the next #601 slice tackles the remaining session-turn.tsx over-500 debt.

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(ui): extract session turn diffs #677

Human Review Status

Pending. A human should make the final merge decision after reviewing the final diff and verification evidence.

Review Focus

• Confirm the extracted panel preserves the existing data-slot names, action confirmation behavior, file expansion behavior, and open/show-in-folder callbacks.
• Confirm the legacy diff summary still appears when turnChanges exists but has no visible entry for the current turn.
• Confirm the turn-change file expansion state remains owned by SessionTurn so it survives panel hide/show transitions.
• Confirm visibleTurnChange is initialized after working so initial renders with turn-change data cannot hit a Solid memo temporal-dead-zone error.
• Confirm this PR does not touch assistant content rendering or product behavior beyond the behavior-preserving extraction and review fixes.

Risk Notes

Behavior is intended unchanged. Main risk is a UI wiring regression in turn-change file rows, legacy diff fallback visibility, or the two-click undo/redo confirmation because the panel moved to a separate component. No dependency, persistence, platform, or public package export change.

Architecture Boundary

Owner lane: #601 message flow.

Base/depends on: dev. #677 depends on this PR.

Touched files:

• packages/ui/src/components/session-turn.tsx
• packages/ui/src/components/session-turn-changes.ts
• packages/ui/src/components/session-turn-changes-panel.tsx
• packages/ui/src/components/session-turn-parent.test.ts

Architecture effect:

• owner extracted
• session-turn.tsx reduced in this slice but remains over 500 LOC for later [Task] UI rewrite v2 Area A: message flow modular rollout #601 work
• no new public package export or dependency change

Review Follow-up

• CodeRabbit legacy diff fallback thread fixed in e11b0cbce and resolved. The fallback now gates on hasVisibleTurnChanges(turnChange()) instead of the presence of the whole turnChanges map.
• Fresh-eyes P3 expansion-state finding fixed in b0b187e35; turn-change expansion state is owned by SessionTurn and passed into the extracted panel.
• Fresh-eyes P1 Solid memo initialization finding fixed in d50f897bd; visibleTurnChange is declared after working.
• Added focused regression assertions covering the fallback gate, expansion-state ownership, and memo declaration order.

How To Verify

bun install --frozen-lockfile: ok in new worktree
Focused UI tests: bun test src/components/session-turn-turn-changes.test.ts src/components/session-turn-parent.test.ts -> 7 pass, 0 fail
bun run typecheck -> 8 successful, 8 total
Targeted perf gate: 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: rerunning after d50f897bd

Screenshots or Recordings

Not included. This PR is intended as a behavior-preserving component extraction with no visual or copy changes; 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 the declared stack base, and my PR title and commit messages use Conventional Commits in English

[coderabbitai]

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: extracting the SessionTurn turn-changes panel into a new component.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description is comprehensive and well-structured, following the template with all major sections completed including Summary, Why, Related Issue, Human Review Status, Review Focus, Risk Notes, Architecture Boundary, How To Verify, and Checklist.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/session-turn-change-panel

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[gemini-code-assist]

Code Review

This pull request refactors the SessionTurn component by extracting the turn-change panel logic and UI into a new standalone component, SessionTurnChangesPanel. This change includes the creation of the new component file, updating type definitions in session-turn-changes.ts, and cleaning up session-turn.tsx by removing redundant logic and state. The architecture manifest was also updated to reflect this extraction. I have no feedback to provide as there were no review comments.

[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	32 -> 24 (-8)	64 -> 40 (-24)	65 -> 63 (-2)	15 -> 13 (-2)	33.3 -> 33.3 (0)	166.7 -> 116.8 (-49.9)	2 -> 3 (+1)	0 -> 0 (0)	pass
default / long-session-input-lag	48 -> 48 (0)	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 -> 48 (0)	64 -> 64 (0)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.8 (+0.1)	33.4 -> 16.8 (-16.6)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-call-expand	16 -> 16 (0)	24 -> 16 (-8)	0 -> 0 (0)	0 -> 0 (0)	16.8 -> 16.7 (-0.1)	16.8 -> 16.7 (-0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-default-open-heavy-bash	24 -> 32 (+8)	24 -> 32 (+8)	64 -> 65 (+1)	14 -> 17 (+3)	50 -> 50 (0)	99.9 -> 116.7 (+16.8)	3 -> 3 (0)	0 -> 0 (0)	pass
default / terminal-side-panel-open	48 -> 48 (0)	56 -> 56 (0)	0 -> 0 (0)	0 -> 0 (0)	33.4 -> 33.3 (-0.1)	33.4 -> 33.3 (-0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / session-scroll-reading	24 -> 32 (+8)	32 -> 32 (0)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.8 (+0.1)	16.7 -> 16.8 (+0.1)	0 -> 0 (0)	0.505 -> 0.505 (0)	warn: cls
low-end / session-scroll-reading-long	72 -> 56 (-16)	96 -> 56 (-40)	90 -> 73 (-17)	63 -> 38 (-25)	16.8 -> 16.8 (0)	233.3 -> 166.7 (-66.6)	4 -> 3 (-1)	0.016 -> 0.011 (-0.005)	pass
low-end / session-timeline-recompute	120 -> 120 (0)	136 -> 136 (0)	108 -> 106 (-2)	178 -> 175 (-3)	100 -> 100 (0)	183.3 -> 183.3 (0)	4 -> 3 (-1)	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 -> 16.7 (0)	16.8 -> 16.8 (0)	0 -> 0 (0)	0 -> 0 (0)	pass

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/ui/src/components/session-turn.tsx`:
- Around line 450-451: The current visibility gate uses props.turnChanges ===
undefined which hides the legacy diff fallback when turnChanges exists but lacks
this turn; update the condition to check the per-turn entry instead. Replace the
leading check props.turnChanges === undefined with a check that the per-turn
entry is missing (e.g., turnChange() == null or !turnChange()) so the whole
condition becomes something like: when={ (turnChange() == null ||
(turnChange()?.files.length ?? 0) === 0) && edited() > 0 && !working() }; keep
references to the existing helpers turnChange(), edited(), and working() when
making the change.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 224ee396-19bd-46a3-ba20-7c2e7c14a063

📥 Commits

Reviewing files that changed from the base of the PR and between 03140f7 and f0958b4.

📒 Files selected for processing (3)

• packages/ui/src/components/session-turn-changes-panel.tsx
• packages/ui/src/components/session-turn-changes.ts
• packages/ui/src/components/session-turn.tsx

[github-actions]

Suggested priority: P2 (includes non-doc, non-test paths outside the low-risk bucket).

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.