Avoid forced layout when measuring the Chat view title

[Dmitriy Vasyura (dmitrivMS)]

Background

This change comes from a Windows startup performance investigation across published Insiders builds and subsequent packaged CPU profiling.

A warmed renderer profile showed repeated synchronous calls to ChatViewTitleControl.getHeight() consuming about 20-30 ms during startup. getHeight() read titleContainer.offsetHeight, which could force Blink style/layout after the title or its toolbars changed.

Observed startup caller paths included:

ChatViewTitleControl.getHeight
<- ChatViewTitleControl.updateTitle
<- ChatViewTitleControl.doUpdate
<- ChatViewTitleControl.update
<- ChatViewPane.showModel

and:

ChatViewTitleControl.getHeight
<- ChatViewPane.layoutChatAndSessions
<- ChatViewPane.doLayoutBody
<- ViewPane layout

The title height is dynamic, but the browser already exposes that state asynchronously and before paint through ResizeObserver.

Change

• Observe the Chat title container's border-box height.
• Cache the most recently reported height.
• Fire the existing onDidChangeHeight event only when the observed height changes.
• Return the cached value from getHeight() instead of reading offsetHeight.
• Keep the existing ChatViewPane relayout path, which applies the final observed size before paint.
• Make the observer factory injectable so unit tests use a real-interface fake instead of replacing the global ResizeObserver.

There is no intended visual or accessibility change.

Startup improvement

Focused warmed CPU traces:

Metric	Before	After
ChatViewTitleControl.getHeight self-time	29.5 ms	0 ms

End-to-end validation used fully bundled current-source Windows desktop applications with identical Electron, resources, extensions, minification, and NLS processing. The applications differed only by this change.

Two independent campaigns ran 30 interleaved measured pairs each, alternating A/B and B/A order after three warmups.

Campaign	Median startup change	Bootstrap 95% interval
First 30 pairs	-9.5 ms	-27.5 ms to +4.0 ms
Repeat 30 pairs	-11.5 ms	-19.5 ms to -1.0 ms
Combined 60 pairs	-11.5 ms	-19.0 ms to -1.0 ms

The conservative end-to-end startup gain is 11.5 ms on this Windows host.

Repeated phase improvements across both campaigns:

Startup phase	Combined paired change	Bootstrap 95% interval
Restore primary viewlet	-13.0 ms	-14.0 ms to -11.0 ms
Restore auxiliary bar	-13.0 ms	-14.0 ms to -11.0 ms
Workbench create/restore	-5.0 ms	-8.0 ms to -3.0 ms
Restore editors	-4.0 ms	-8.0 ms to -1.0 ms
Renderer start to workbench ready	-7.0 ms	-11.0 ms to 0.0 ms

The phase measurements overlap and should not be summed.

Improvements to other scenarios

The removed forced layout was not startup-specific. The same code runs when:

• A Chat model is shown or switched.
• A session title/custom title changes.
• A request updates a title.
• Chat view actions or navigation controls change size.
• Chat is restored in the primary sidebar.
• Chat is restored in the auxiliary bar.
• The containing pane is resized or moved between view locations.

These paths now consume the cached browser-reported height instead of synchronously flushing style/layout. ResizeObserver continues to trigger the existing relayout event whenever the real border-box height changes.

The measured 13 ms improvements in both primary viewlet and auxiliary-bar restoration demonstrate that the gain applies across multiple Chat view locations.

Compatibility and behavior

• The title remains dynamically sized.
• Visibility changes are reflected by the observer, including a height of zero.
• Duplicate height notifications do not trigger redundant relayouts.
• Final layout is applied before paint through the existing onDidChangeHeight listener.
• The observer is disconnected with the control's disposables.
• The observed element belongs to the same window as the Chat view, so auxiliary-window behavior is unchanged.
• Keyboard navigation, ARIA state, labels, and focus behavior are unchanged.

The focused test verifies:

1. An observed height updates getHeight().
2. The existing height-change event fires.
3. Repeating the same height does not fire another event.
4. Observing zero updates the cache and fires the second real change.

Validation

npm run typecheck-client
npm run transpile-client
scripts\test.bat --run src\vs\workbench\contrib\chat\test\browser\widgetHosts\viewPane\chatViewTitleControl.test.ts --run src\vs\workbench\contrib\chat\test\browser\widget\chatWidget.test.ts

• Client typecheck passed.
• Client transpilation passed.
• 16 focused Chat tests passed.
• Production desktop baseline/fix bundles succeeded.
• Packaged startup succeeded.
• Two independent correctness/diff reviews found no significant issue.

Rejected alternatives

• Coalescing title-bar overflow checks did not reduce the targeted CPU or forced-layout cost.
• Replacing toolbar getBoundingClientRect() reads with cached observer width removed that leaf but did not improve packaged startup over 30 interleaved pairs.

[Copilot]

Pull request overview

Replaces synchronous Chat title height reads with an asynchronously updated cache to reduce forced layout during startup and relayout.

Changes:

• Adds ResizeObserver-based title height tracking.
• Relayouts Chat when the cached height changes.
• Adds focused height-change tests.

Show a summary per file

File	Description
chatViewTitleControl.ts	Adds observer-backed height caching.
chatViewPane.ts	Uses the updated title control constructor.
chatViewTitleControl.test.ts	Tests cached height and deduplication.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Balanced

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced