Batch action list width measurements

[ulugbekna]

Summary

• batch action-list row width writes, geometry reads, and restores into three phases
• preserve the row-to-item mapping used for toolbar-width accounting in visible and temporarily expanded hidden-item paths
• add regression coverage for measurement ordering, toolbar accounting, and width restoration

Performance

Chromium CDP instrumentation over 49 representative rows and 200 measurement passes:

Implementation	Layouts	Style recalculations	Duration
Alternating write/read/write	9,801	9,801	110.2 ms
Batched write/read/write	2	201	7.5 ms

Measured row widths were identical.

Validation

• npm run typecheck-client -- --pretty false
• ./scripts/test.sh --run src/vs/platform/actionWidget/test/browser/actionList.test.ts
• npm run valid-layers-check
• npm run precommit

Fixes #326640

cc @sandy081

[Copilot]

Pull request overview

Batches action-list width measurements to reduce layout thrashing while preserving toolbar accounting and row styles.

Changes:

• Groups width writes, geometry reads, and restoration into phases.
• Adds regression coverage for ordering, toolbar width, and restoration.

Show a summary per file

File	Description
actionList.ts	Adds batched row-width measurement helper.
actionList.test.ts	Tests batching and width calculations.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Medium

[ulugbekna]

@sandy081 merging to get more selfhosting time in. We can revert if you don't like the change when you're back