fix(agent-manager): improve inspector resize performance

[marius-kilocode]

Problem

Resizing the Agent Manager inspector repeatedly committed layout changes while the diff viewer and mounted xterm instances reflowed. Large diffs and terminal scrollback could produce visible stalls during a drag.

Change

• Limit inspector width commits to approximately 30 FPS while preserving the latest width for a trailing update.
• Skip ResizeObserver-driven xterm refits for hidden terminal tabs. Hidden terminals refit when activated at their current dimensions.
• Add regression coverage for both resize invariants.

Measured Improvement

Measured with the VS Code self-test profiler using the same 1.46-second divider drag, 90 synthetic pointer updates, and the same Agent Manager webview scenario. Diff and terminal runs were captured separately. The terminal scenario used 5,000 lines of scrollback.

Inline Diff Resize

Metric	Before	After	Change
Total RunTask time	583.72 ms	395.38 ms	-32.3%
Longest task	69.74 ms	14.60 ms	-79.1%
Tasks over 16 ms	2	0	-100%
Tasks over 50 ms	1	0	-100%
UpdateLayoutTree	124.33 ms	77.60 ms	-37.6%
Layout	40.86 ms	31.88 ms	-22.0%
Paint	46.57 ms	32.40 ms	-30.4%

Inline Terminal Resize

Metric	Before	After	Change
Total RunTask time	480.59 ms	288.42 ms	-40.0%
Longest task	7.65 ms	9.96 ms	+30.3%
Tasks over 16 ms	0	0	unchanged
Tasks over 50 ms	0	0	unchanged
UpdateLayoutTree	53.99 ms	23.20 ms	-57.0%
Layout	37.70 ms	22.80 ms	-39.5%
Paint	27.16 ms	17.11 ms	-37.0%
FunctionCall	103.13 ms	47.10 ms	-54.3%

The terminal after-run longest task remained below 10 ms. The one-time post-build capture was discarded from the comparison because it included an unrelated outer VS Code terminal-panel height transition.

User-visible Result

• Diff resizing no longer produces the measured long main-thread stalls.
• Terminal scrollback remains responsive while the inspector is dragged.
• Inactive terminal tabs avoid unnecessary scrollback reflow and still fit correctly when selected.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

lgtm. The resize throttle in resizeSide guarantees a trailing commit (the flush loop only exits after applying the latest pendingSideWidth), and the debounced persistLocalTabs effect observes the throttled signal, so the final width always persists. The hidden-terminal ResizeObserver skip is safely paired with the existing activation repaint path (createEffect → scheduleRepaint → runRepaint → fit.fit() + syncSize()), which refits at current dimensions on tab activation. No new listeners, timers, or unbounded rAF chains — no memory-leak concerns introduced. Regression tests follow the established source-invariant convention of the test file, and a user-facing changeset is included.

Files Reviewed (4 files)

• .changeset/sync-inspector-width.md
• packages/kilo-vscode/tests/unit/agent-manager-terminal-layout.test.ts
• packages/kilo-vscode/webview-ui/agent-manager/AgentManagerApp.tsx
• packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx

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_{Reviewed by kimi-k3 · Input: 57.8K · Output: 11.4K · Cached: 405.5K}

_{Review guidance: REVIEW.md from base branch main}