Avoid synchronous activity bar size measurement

[Dmitriy Vasyura (dmitrivMS)]

Remove the vertical activity bar's synchronous global-action-container clientHeight read during startup layout.

• Compute global action height from the rendered action count and the existing action-height/gap inputs that also drive CSS, including gaps only between items.
• Recompute available view-switcher space when Accounts is hidden or shown; keep repeated visibility updates idempotent.
• Preserve classic, compact, and Modern UI sizing, horizontal sidebar bars, and keyboard behavior. No CSS, labels, or keybindings change.

This follows the remaining activity-bar layout cost observed after the titlebar fix in #334751.

Performance validation

Compared baseline and candidate on the same complete Code OSS development build at 333552ca38d5e15c8b734157159fcfcae9553c87, using separate warmed profiles and alternating order. Baseline modules were compiled from HEAD using the repository's normal compiler; only the three affected generated modules were exchanged.

Five measured untraced launches and three measured traced launches per variant, after warmups:

Startup metric (median)	Baseline	Candidate
Activity-bar layout sampled self time	26.094 ms	0 ms
Total forced style/layout before didStartWorkbench	26.850 ms	14.936 ms
Untraced startup	4,147 ms	4,170 ms
Traced startup	6,352 ms	6,352 ms

Forced layout decreases by approximately 44.4% (11.9 ms) rather than merely disappearing from the original function's profile. A remaining flush occurs during focus handling.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation matches its stated performance goal and includes focused regression coverage.

Review tier: Balanced
Findings: None

What changed in this PR

Replaces startup DOM measurement with deterministic activity-bar sizing while preserving existing layouts and interactions.

Changes:

• Calculates global action height from action count, height, and gap.
• Relayouts when Accounts visibility changes.
• Adds coverage across activity-bar modes and orientations.

File	Description
globalCompositeBar.ts	Adds calculated height and change events.
activitybarPart.ts	Uses calculated global-action sizing.
sidebarPart.ts	Adapts to the revised constructor contract.
globalCompositeBar.test.ts	Tests sizing and visibility idempotence.
activitybarPart.test.ts	Tests layout modes without DOM measurement.

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[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

Benjamin Christopher Simmonds (@benibenj)

Matched files:

• src/vs/workbench/browser/parts/activitybar/activitybarPart.ts
• src/vs/workbench/browser/parts/globalCompositeBar.ts
• src/vs/workbench/browser/parts/sidebar/sidebarPart.ts