chat: Avoid transient sticky row rebuilds during resize

[Benjamin Christopher Simmonds (benibenj)]

Part of #339863; this does not resolve the whole multi-pane resizing issue.

Summary

Width layout clears the cached sticky source geometry, but previously deferred its measurement until the next animation frame. The existing tree refresh could therefore see estimated geometry, rebuild the sticky row, and then rebuild it again once the measured geometry arrived.

Run the existing source-range measurement synchronously before that tree refresh. This is one production method-call change: no generic-tree changes, new caching/lifetime machinery, debounce, setting, or skipped measurement. The existing width guard and content/source invalidation paths remain in place.

Add regression coverage for unchanged/height-only layout, width measurement and sticky/editor identity, focus/scroll anchors, and streaming/hidden updates using real code-block editors.

Validation

• npm run transpile-client - passed.
• ./scripts/test.sh --run src/vs/base/test/browser/ui/tree/objectTree.test.ts --run src/vs/workbench/contrib/chat/test/browser/widget/chatListWidget.test.ts --run src/vs/workbench/contrib/chat/test/browser/widget/chatListRenderer.test.ts - 832 passing, one pre-existing pending.
• ./scripts/test.sh --run src/vs/workbench/contrib/chat/test/browser/widget/chatListWidget.test.ts --grep "width changes keep sticky templates and code editors while updating measured geometry" - fails on the original baseline as expected; passes with this fix.
• npm run typecheck-client - passed.
• npm run eslint -- src/vs/workbench/contrib/chat/browser/widget/chatListRenderer.ts src/vs/workbench/contrib/chat/test/browser/widget/chatListWidget.test.ts - passed.
• git --no-pager diff --check - passed.

Real current-source Code OSS Agents windows, inspected with @hediet/dbgjs, also passed streaming while user-scrolled, hidden updates, follow-tail, sticky transitions, sash dragging, font/zoom changes, code/diff model identity, drafts, focus, and removal/disposal checks.

Native measurements

Compared base 564de3cb8366c7a23c2a1235ff3e8597566b4075, an earlier broader optimization, and this smaller fix. Three fresh native lifecycles per variant in balanced order; 1/3/6 populated panes, eight Markdown/TypeScript-code-block turns per pane, and combined/height-only/width-only resizing: 81 scenarios. Resizes use real BrowserWindow.setBounds. Complete lifecycles were serialized; scored timings use no profiler, trace, or operation-counting wrappers.

Six-pane medians:

Metric	Base	Earlier broad patch	This one-call fix
Combined: browser layouts / delivered resize	131.32	119.43	119.34
Combined: p95 frame gap	84.2 ms	66.7 ms	66.8 ms
Width-only: browser layouts / delivered resize	128.45	114.95	114.19
Width-only: p95 frame gap	67.0 ms	43.5 ms	43.2 ms

The fixed combined p95 ranged from 66.7 to 66.9 ms across the three runs. Separate dbgjs counting found zero sticky row/template creations or disposals in the fixed six-pane combined resize run, versus about 1.29 per chat layout on the baseline.

These are local development-build observations, not statistical guarantees. Height-only work is essentially unchanged, and significant style/layout cost remains. No paid provider orchestration was involved. All temporary app/debugger processes and profiles were cleaned up.

[github-actions]

Screenshot Changes

Base: 6ffd78e1 Current: 6c538411

Changed (1)

sessions/chatCompositeBar/MixedStatuses/Light

Before	After

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused fix is consistent with the layout lifecycle and is covered by comprehensive regression tests.

Review effort: Balanced
Findings: None

What changed in this PR

Synchronizes sticky-row geometry measurement during chat width changes to prevent transient rebuilds while preserving focus and editor identity.

Changes:

• Refreshes invalidated sticky source ranges before the tree refresh.
• Adds regression coverage for resizing, streaming, visibility, focus, scrolling, and editor retention.

File	Description
chatListRenderer.ts	Measures sticky source geometry synchronously on width changes.
chatListWidget.test.ts	Adds real-editor sticky-content retention tests.

---

💡 Configure MCP servers for context-aware, tailored reviews. Learn more in the docs.