fix(web): diff panel keeps the chosen scope while a turn runs

[juliusmarminge]

Problem

Selecting Uncommitted (or a turn) in the Diff panel snapped back to Changes within a second or two while the agent was working, and again at the end of each turn. Reported with a screen recording: the user re-picked Uncommitted four or five times in 26 seconds and it reverted each time.

Cause

ChatView reset the scope to Changes in a layout effect meant for generic openings: the panel opening, or a thread switch. That effect was keyed on activeThreadRef, which was memoized on the whole thread shell. The shell is replaced on every runtime update during a run, so the ref got a new identity, the effect re-ran, and the scope was reset each time.

Separately, the proactive "open the diff when a turn completes" path also called selectGitScope("branch") when the diff was already the active surface.

Fix

• useActiveThreadRef keys the ref on the environment and thread ids, so it stays stable for as long as the view shows the same thread. Thread switches, environment switches and draft promotion behave as before.
• resolveProactiveTurnDiffAction returns ignore when the diff is already the active surface. A diff tab hidden behind another tab is still brought forward.

Verification

• useActiveThreadRef.test.tsx fails when the memo is keyed on the thread object again and passes with the fix. ChatView.logic.test.ts covers the proactive guard (130 tests pass).
• Web typecheck is clean.
• Adversarially reviewed by GPT 6.1 Sol and Claude Fable 5.1: no stale consumers across the hooks that depend on the ref, and no other path that resets the scope mid-turn.
• Not verified in a running client; no UI change beyond the scope no longer resetting.

Related perf work in the same panel: #16033.

Implementation: Claude Opus 5.5 via Claude Code in T3 Code; reviews by GPT 6.1 Sol (Codex) and Claude Fable 5.1.

🤖 Generated with Claude Code

Fixes #15352

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[macroscopeapp]

Approvability

Verdict: Approved at 9ff600c

Macroscope's review found this PR approvable — This is a focused web bug fix that preserves the active diff panel and chosen scope during turn updates. The new hook is small, identity-keyed by stable thread IDs, and covered by tests, with no changes to defaults, schemas, infrastructure, or static-analysis settings.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between 7bf6de1 and 9ff600c.

📒 Files selected for processing (5)

• apps/web/src/components/ChatView.logic.test.ts
• apps/web/src/components/ChatView.logic.ts
• apps/web/src/components/ChatView.tsx
• apps/web/src/hooks/useActiveThreadRef.test.tsx
• apps/web/src/hooks/useActiveThreadRef.ts

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📝 Walkthrough

Walkthrough

The changes make proactive diff actions ignore active diff surfaces and add a memoized hook for deriving the active thread reference. ChatView uses the hook, with tests covering both changes.

Changes

Proactive diff action

Layer / File(s)	Summary
Ignore actions on diff surfaces apps/web/src/components/ChatView.logic.ts, apps/web/src/components/ChatView.logic.test.ts	resolveProactiveTurnDiffAction now returns "ignore" when the active surface is a diff. A test covers a ready checkpoint with three modified files.

Active thread reference

Layer / File(s)	Summary
Derive and use the scoped thread reference apps/web/src/hooks/useActiveThreadRef.ts, apps/web/src/components/ChatView.tsx, apps/web/src/hooks/useActiveThreadRef.test.tsx	The new useActiveThreadRef hook derives and memoizes a scoped reference from the environment and thread IDs. ChatView uses the hook. Tests cover reference identity across updates, changed thread IDs, and absent threads.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to 9ff60

The change is ready to merge after normal checks. The selected diff scope should remain in place during thread updates and when a turn completes.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the main change: the Diff panel keeps the chosen scope while a turn runs.
Description check	✅ Passed	The description clearly explains the problem, cause, fix, and verification. It references the related issue and identifies what was not verified. It does not explicitly address maintainer approval or …

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.9 KiB	0 B (0.0%)	29.3 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 7bf6de1 · PR result: 9ff600c · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.