perf(web): huge-thread switch no longer blanks the chat pane

[juliusmarminge]

Switching between large conversations briefly cleared the chat pane. This keeps the timeline mounted, remembers the latest painted content for up to 16 threads, and refreshes the list and scroll position when its displayed thread changes. Remembered content from another thread is paint-only until the requested thread is ready.

Rebased all 12 original commits onto #11198, the fourth layer of the performance stack. Two compatibility commits correct optional callback props and complete test fixtures for current types.

Direct production-browser comparison

Six counterbalanced rounds per build, identical isolated data and browser. Streaming values are median synchronous update milliseconds; navigation is the median time to the correct visible final message.

Metric	Main	Four-PR stack	Stack + this PR
Prose after completed code	18.575 ms	14.700 ms	14.800 ms
Growing code block	174.425 ms	114.775 ms	111.775 ms
Cached thread switch	252.625 ms	252.475 ms	178.500 ms
Switches with blank frames	48/48	48/48	0/48

This PR reduces cached switch latency 29.3% versus its parent and removes the observed blank pane. All 144 measured switches reached the correct destination. Separate rapid navigation checks in light and dark themes also passed with no sampled blank frames.

Across the fixed 960-update streaming mix, the four-PR stack takes 32.0% less synchronous update time than main; adding this PR makes that 33.2% less. Its incremental streaming difference varies across rounds and is not a reliable additional streaming win. These are local web measurements, not provider throughput or cold/remote/mobile timings.

Full methods, exact revisions and results · Raw samples and scripts

Browser evidence

Recordings were captured separately from timing and preserve screenshot timestamps.

Before: four-PR stack

Before recording

After: stack plus this PR

After recording

Validation

• Three focused timeline/scroll test files passed after rebase: 192 tests. After compatibility fixes, affected suites passed (184 tests), followed by 137 passing ChatView logic tests after the final fixture-only correction.
• Final web typecheck and production build passed.
• Browser verification used two 400-message synthetic threads, normal app pagination, six rounds and 48 cached switches per build, plus rapid interrupted navigation in both themes.
• Web verified. Desktop shares the web implementation, but Electron keybindings were not exercised; mobile is unchanged and was not tested.

Original implementation: cursor-grok-4.6-high-fast / Cursor. Rebase, compatibility fixes and measurements: GPT-6 / Codex with agent-browser.

  

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.0 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.6 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	57.1 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	10	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.8 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	8	—	21	✅

Baseline: unavailable · PR result: beb61e4 · 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: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

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📝 Walkthrough

Walkthrough

ChatView resolves remembered timelines by thread and environment, preserves displayed content during thread switches, and disables active-thread actions for paint-only timelines. MessagesTimeline scopes rendering state to the displayed thread. Tests cover fallback precedence and isolation.

Changes

Thread timeline switching

Layer / File(s)	Summary
Timeline cache and resolution apps/web/src/components/ChatView.logic.ts, apps/web/src/components/ChatView.logic.test.ts	Timeline storage retains workspace metadata. Fallback requires loading for remembered active-thread entries and matching environments for cross-thread entries. Tests cover empty-thread, cross-environment, persistence, precedence, and ephemeral URL cases.
ChatView displayed timeline integration apps/web/src/components/ChatView.tsx	ChatView preserves remembered content and workspace context during switches, identifies paint-only timelines, disables active-thread operations, and scrolls when the active timeline becomes displayed.
Displayed-thread rendering state apps/web/src/components/chat/MessagesTimeline.tsx	Timeline state, row projection, refresh identity, and structural sharing use displayThreadKey. Hidden empty states render a full-height background surface.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant ChatView
  participant ChatViewLogic
  participant MessagesTimeline
  ChatView->>ChatViewLogic: Resolve timeline for active thread
  ChatViewLogic-->>ChatView: Return displayed entries and thread key
  ChatView->>MessagesTimeline: Render entries with displayThreadKey
  MessagesTimeline->>MessagesTimeline: Scope state to displayed thread
  ChatView->>MessagesTimeline: Disable paint-only actions when threads differ

Loading

Suggested reviewers: t3dotgg, maria-rcks, bil0000

Merge Risk: 🟡 Moderate · up to dc279

Thread switching can display incorrect or empty timeline content during loading transitions. These navigation regressions should be resolved before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ 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 and concisely describes the primary change: preventing blank chat-pane flashes when switching between large threads.
Description check	✅ Passed	The description explains what changed, why it changed, UI behavior, validation results, browser evidence, and test coverage. It does not use the template headings or include an explicit checklist, but…

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Actionable comments posted: 3

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@apps/web/src/components/ChatView.logic.ts`:
- Line 307: Update the held-timeline selection logic around held.threadKey and
input.activeThreadKey so a held timeline is allowed when its thread is active
again, including the A → B → A loading sequence with empty timelineEntries.
Remove the thread-key inequality while preserving other guards, and add a
regression test covering this navigation sequence and ensuring MessagesTimeline
receives the held entries.
- Around line 278-294: Update the held timeline flow around
rememberReadyThreadTimeline, peekHeldThreadTimeline, resetHeldThreadTimeline,
and resolveThreadSwitchTimeline so a cached timeline cannot cross account or
environment boundaries. Scope heldThreadTimeline by the active
account/environment and reject mismatches, or clear it before rendering a
different context, while preserving reuse within the same context.

In `@apps/web/src/components/ChatView.tsx`:
- Line 8394: Update the loading branch around displayedTimeline and
MessagesTimeline so held entries retain the complete timeline context they were
created with, including routeThreadKey, environment, turn state, checkpoints,
and callbacks, and pass that context to MessagesTimeline instead of the current
thread context. Ensure attachment, diff, citation, and revert handling remains
bound to the displayed timeline; otherwise defer rendering held entries until
the matching thread context is ready.

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Actionable comments posted: 1

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Inline comments:
In `@apps/web/src/components/ChatView.logic.ts`:
- Around line 352-354: Gate the rememberedForActive fallback in the timeline
resolver so it returns cached entries only when input.loading is true; preserve
the existing non-empty check and active thread key. Add a regression test
covering loading: false with nextEntries: [] to verify an empty resolved thread
does not return remembered entries.

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Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This production web change replaces thread-view remounts with cached timeline snapshots and in-place virtualization updates across ChatView and MessagesTimeline, affecting rendering, scrolling, and interaction state. Its cross-component runtime scope and an unresolved review concern around A→B→A loading make human review appropriate.

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Actionable comments posted: 1

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@apps/web/src/components/ChatView.tsx`:
- Around line 3362-3366: Move the rememberReadyThreadTimeline call out of the
ChatView render path and into a post-commit effect, preserving the
!threadDetailLoading, non-empty timelineEntries, and
!timelineHasEphemeralPreviewUrls readiness checks. Add a regression test
covering an abandoned ready render to ensure its timeline is not cached or shown
by a later loading render.

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