2nd/10_Wiki/Topics/Frontend/리플로우 및 리페인트(Reflow and Repaint).md

---
id: wiki-2026-0508-리플로우-및-리페인트-reflow-and-repaint
title: 리플로우 및 리페인트 (Reflow and Repaint)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Reflow, Repaint, Layout, Paint, 브라우저 렌더링]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [browser, rendering, performance, frontend]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: javascript
  framework: browser
---

# 리플로우 및 리페인트 (Reflow and Repaint)

## 매 한 줄
> **"매 Reflow = geometry 재계산, Repaint = pixel 재그리기"**. Reflow 매 layout phase — DOM/CSS geometry 변화 시 발생 — 매 expensive. Repaint 매 paint phase — color/visibility 변화 — 매 cheaper. 매 60fps 의 핵심 = 매 둘 다 피하기 + 매 compositor-only 속성 사용.

## 매 핵심

### 매 Pixel Pipeline
1. JS — DOM 변경.
2. **Style** — selector 매칭, computed style.
3. **Layout** (= Reflow) — geometry 계산.
4. **Paint** (= Repaint) — pixel 채우기.
5. **Composite** — GPU layer 합성.

### 매 Reflow trigger
- DOM 추가/제거 (block-level).
- `width`, `height`, `top`, `left`, `padding`, `margin`, `border`.
- `font-size`, `text-align`.
- `offsetWidth`, `getBoundingClientRect()` 읽기 — 매 force sync layout.
- viewport resize.

### 매 Repaint-only trigger
- `color`, `background-color`, `visibility`, `outline`.

### 매 Compositor-only (best)
- `transform`, `opacity`, `filter` (with `will-change`).
- 매 main thread 의 X — 매 GPU layer 만.
- 매 60fps 의 핵심.

### 매 Layout Thrashing
- 읽기/쓰기 alternate — 매 매번 force reflow — 매 worst.
- batch 매 read 다음 batch 매 write — 매 1회 reflow.

## 💻 패턴

### Compositor-only animation
```css
/* 매 GOOD — transform/opacity 만 — GPU */
.box {
  transition: transform 0.3s, opacity 0.3s;
  will-change: transform;  /* 매 hint — 매 layer 미리 promote */
}
.box:hover {
  transform: translateX(20px) scale(1.1);
  opacity: 0.8;
}

/* 매 BAD — top/left — 매 reflow */
.box-bad:hover {
  top: 20px;
  left: 20px;
}
```

### Read-write batching
```javascript
// 매 BAD — thrashing
items.forEach(el => {
  el.style.width = el.offsetWidth + 10 + 'px';  // read + write
});

// 매 GOOD — batch
const widths = items.map(el => el.offsetWidth);  // all reads
items.forEach((el, i) => {
  el.style.width = widths[i] + 10 + 'px';  // all writes
});
```

### DocumentFragment for bulk insert
```javascript
const frag = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
  const li = document.createElement('li');
  li.textContent = `Item ${i}`;
  frag.appendChild(li);  // 매 detached — 매 reflow 의 X
}
document.querySelector('ul').appendChild(frag);  // 매 1회 reflow
```

### `will-change` (정확)
```css
/* 매 hover 직전 추가, 끝 후 제거 */
.card { will-change: auto; }
.card:hover { will-change: transform; }
/* 매 항상 will-change — 매 anti-pattern — 매 메모리 낭비 */
```

### CSS Containment
```css
.widget {
  contain: layout paint;  /* 매 외부 영향 차단 — 매 reflow 매 widget 내부 만 */
}
```

### requestAnimationFrame
```javascript
function animate() {
  // read
  const top = el.getBoundingClientRect().top;
  // write (next frame)
  requestAnimationFrame(() => {
    el.style.transform = `translateY(${top + 10}px)`;
  });
}
```

### content-visibility (skip rendering)
```css
.below-fold {
  content-visibility: auto;
  contain-intrinsic-size: 0 500px;  /* 매 placeholder size */
}
```

## 매 결정 기준
| 변경 속성 | 비용 |
|---|---|
| `transform`, `opacity` | Composite only (cheapest) |
| `color`, `background` | Repaint |
| `width`, `height`, `top`, font | Reflow (expensive) |
| DOM insert/remove | Reflow (expensive) |
| read `offsetWidth` after write | Force sync layout (worst) |

**기본값**: animation 매 `transform` + `opacity` 만, bulk DOM 매 fragment, read/write batch.

## 🔗 Graph
- 부모: [[브라우저 렌더링]] · [[Critical Rendering Path (CRP)|Critical Rendering Path]]
- 변형: [[Layout_Thrashing|Layout Thrashing]]
- 응용: [[Web Performance]]

## 🤖 LLM 활용
**언제**: jank debugging, animation performance, "왜 느림" 분석.
**언제 X**: 매 specific browser bug — 매 DevTools Performance 매 더 정확.

## ❌ 안티패턴
- **`will-change` 전체**: 매 메모리 낭비 — 매 hover 직전 만.
- **layout thrashing**: 매 loop 안 read-write alternate.
- **`top/left` animation**: 매 reflow — 매 `transform` 의 사용.
- **DOM insert in loop**: 매 fragment 의 사용.
- **`offsetWidth` in RAF callback after write**: 매 force layout.

## 🧪 검증 / 중복
- Verified (web.dev Rendering Performance, MDN, Chrome DevTools docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — pixel pipeline + 매 thrashing 패턴 |