[Jinwoo-H]

Closes #512

Summary

• Debounce terminal refit — Replace RAF-based batching with a 150ms trailing-edge debounce on the ResizeObserver in use-terminal-pane-global-effects.ts. The synchronous xterm scrollback reflow (fitAddon.fit() → terminal.resize()) blocks the renderer for 500ms–2s on Windows with large scrollback buffers; deferring it 150ms lets the layout settle first so the reflow runs once instead of per-frame.
• Parallelize auth cache rebuild — rebuildAuthorizedRootsCache now runs git worktree list + realpath() for all repos concurrently via Promise.all instead of sequentially. With 3 repos on Windows, this cuts wall-clock time from ~2–3s to ~500ms–1s.
• Suppress CSS transition on sidebar mount — Added a isMounting guard that applies transition-none on the sidebar's first render, preventing Chromium from animating width from auto → 320px and triggering intermediate terminal resizes.

Test plan

• Open the right sidebar on Windows with multiple worktrees active and verify the app stays responsive
• Verify the sidebar still animates smoothly when toggling open/close after the initial mount
• Verify terminal dimensions update correctly after sidebar open (type a long line, confirm it wraps at the right column)
• Verify drag-resize of the sidebar still has smooth transitions
• Verify File Explorer loads files correctly (auth cache rebuild still works)
• Run existing test suite — confirm no new failures

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