2nd/10_Wiki/Topics/Frontend/레이아웃 스래싱(Layout Thrashing).md

---
id: wiki-2026-0508-레이아웃-스래싱-layout-thrashing
title: 레이아웃 스래싱 (Layout Thrashing)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Layout Thrashing, Forced Synchronous Layout, FSL]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [performance, rendering, javascript, frontend]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: javascript
  framework: browser
---

# 레이아웃 스래싱 (Layout Thrashing)

## 매 한 줄
> **"매 layout thrashing = read DOM geometry → write style → read DOM 매 반복 — 매 force sync layout 매번"**. 매 브라우저 매 batch reflow 의 X — 매 즉시 reflow 강제 — 매 100x 느림. 매 해결 = 매 read 모두 batch → 매 write 모두 batch.

## 매 핵심

### 매 트리거 (read = force layout)
- `offsetTop/Left/Width/Height`.
- `clientTop/Left/Width/Height`.
- `scrollTop/Left/Width/Height`.
- `getBoundingClientRect()`.
- `getComputedStyle()` (some properties).
- `innerText` (특정 case).

### 매 메커니즘
- 매 write (`el.style.x = ...`) — 매 layout invalidate, 매 brower 매 batch 대기.
- 매 read (`el.offsetWidth`) — 매 latest geometry 필요 — 매 강제 reflow.
- 매 loop 안 read/write alternate — 매 매번 reflow — 매 thrashing.

### 매 해결
1. **Batch**: read 모두 → write 모두.
2. **Cache**: 매 1회 read, 매 변수 저장.
3. **rAF**: read in current frame, write in next frame.
4. **FastDOM** library — read/write queue.
5. **CSS containment**: `contain: layout` — 매 thrashing 매 isolate.

## 💻 패턴

### Anti — 매 thrashing
```javascript
const items = document.querySelectorAll('.item');
items.forEach(el => {
  // 매 read → 매 write → 매 read → 매 write — 매 thrashing
  el.style.width = el.offsetWidth + 10 + 'px';
});
// 매 100 items → 매 100 layouts
```

### Fix — batch
```javascript
const items = document.querySelectorAll('.item');

// 매 phase 1: read 모두
const widths = Array.from(items).map(el => el.offsetWidth);

// 매 phase 2: write 모두
items.forEach((el, i) => {
  el.style.width = widths[i] + 10 + 'px';
});
// 매 1 layout
```

### rAF read/write split
```javascript
function syncSizes() {
  // 매 current frame: read
  const heights = Array.from(rows).map(r => r.offsetHeight);

  requestAnimationFrame(() => {
    // 매 next frame: write
    rows.forEach((r, i) => {
      r.style.minHeight = heights[i] + 'px';
    });
  });
}
```

### FastDOM
```javascript
import fastdom from 'fastdom';

fastdom.measure(() => {
  const w = el.offsetWidth;
  fastdom.mutate(() => {
    el.style.width = w + 10 + 'px';
  });
});
// 매 internally batch — 매 1 layout per frame
```

### ResizeObserver (매 read 필요 X)
```javascript
const ro = new ResizeObserver(entries => {
  for (const entry of entries) {
    const { width, height } = entry.contentRect;  // 매 already computed
    entry.target.dataset.width = width;  // 매 write — 매 layout 의 X
  }
});
ro.observe(element);
```

### CSS Containment (isolate)
```css
.widget {
  contain: layout;  /* 매 widget 내부 변경 — 매 outer 매 reflow 의 X */
}
```

### DevTools 측정
```javascript
performance.mark('layout-start');
heavyOperation();
performance.mark('layout-end');
performance.measure('layout', 'layout-start', 'layout-end');
// 매 DevTools Performance panel — 매 purple "Layout" bar — 매 thrashing 표시
```

### Virtual list (대량 item)
```javascript
// 매 모든 item layout — 매 X
// 매 visible 만 render — 매 react-virtual / @tanstack/virtual
import { useVirtualizer } from '@tanstack/react-virtual';

const rowVirtualizer = useVirtualizer({
  count: 10000,
  getScrollElement: () => parentRef.current,
  estimateSize: () => 40,
});
```

## 매 결정 기준
| 상황 | 접근 |
|---|---|
| Loop 안 size 읽고 변경 | batch read → batch write |
| 비동기 read/write | rAF split |
| Element resize 감지 | ResizeObserver |
| Component-level isolation | `contain: layout` |
| 1000+ rows | Virtualization |
| Library 매 abstraction | FastDOM |

**기본값**: read all → write all + 매 ResizeObserver (size 감지) + 매 Virtual list (대량).

## 🔗 Graph
- 부모: [[리플로우 및 리페인트(Reflow and Repaint)]] · [[Web Performance]]
- 변형: [[Forced Synchronous Layout]]
- Adjacent: [[ResizeObserver]]

## 🤖 LLM 활용
**언제**: jank 디버깅, "왜 scroll 느림", DevTools Performance 분석.
**언제 X**: 매 framework-specific (React reconciler) 내부 — 매 framework docs.

## ❌ 안티패턴
- **Loop 안 read+write**: 매 thrashing 의 정의.
- **`offsetWidth` after style write**: 매 force layout.
- **jQuery `.css()` chain**: 매 read/write 매 mix.
- **`scrollTop` polling rAF**: 매 ScrollObserver/IntersectionObserver 의 사용.
- **모든 row layout 매 손수**: 매 virtual list 의 사용.

## 🧪 검증 / 중복
- Verified (web.dev Avoid Layout Thrashing, Paul Irish gist, Chrome DevRel).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — read/write batch + rAF + ResizeObserver |