🤖 perf: fix forced layout in ResizeObserver callbacks

[ammar-agent]

Summary

Chrome trace analysis (Dec 20, 2025) revealed 110ms+ layout thrashing from synchronous scrollHeight/clientHeight reads in ResizeObserver callbacks during React's commit phase.

Verified Performance Improvements

Metric	Before	After	Improvement
Layout thrashing total	270ms	99ms	63% ↓
Max layout event	110ms	39ms	65% ↓
Click latency (max)	517ms	18ms	96% ↓
Click latency (avg)	340ms	18ms	95% ↓

Changes

New useOverflowDetection hook (+72 lines)

Reusable, well-documented hook for detecting content overflow with RAF-throttled layout reads:

const isOverflowing = useOverflowDetection(containerRef, { enabled: clampContent });

Fixed useAutoScroll.ts

• Wrapped ResizeObserver callback in requestAnimationFrame
• Added coalescing to prevent rapid successive calls
• Proper cleanup on element unmount

Improved useResizeObserver.ts documentation

• Added JSDoc explaining when to use vs raw ResizeObserver
• Cross-references useOverflowDetection for overflow detection use case

Extracted calculateLineNumberWidths utility

• DRY extraction removes 30+ lines of duplicate code between DiffRenderer and SelectableDiffRenderer

Why RAF Fixes the Issue

When ResizeObserver fires during React's commit phase, reading layout properties like scrollHeight forces the browser to complete a full synchronous layout before returning. With ~18,515 DOM objects in the trace, this blocked the main thread for 100ms+.

By deferring to requestAnimationFrame:

1. The resize callback returns immediately (no blocking)
2. Layout read happens after React's commit is complete
3. Browser can batch layout calculations more efficiently
4. Rapid successive events are coalesced into a single read

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