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10_Wiki/Topics 대규모 정리: - 오류 캡처/미완성 stub 문서 227개 제거 - 교차폴더 중복 43클러스터 병합 (63파일 → redirect) - 링크명 정규화: 깨진 링크 수정·redirect 직결·개념 매핑 ~2,400건 - 카테고리 MOC 6개 신규 생성 - Graph 섹션 미해결 related-keyword 링크 10,058건 제거 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
161 lines
5.1 KiB
Markdown
161 lines
5.1 KiB
Markdown
---
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id: wiki-2026-0508-성능-최적화-reflow-repaint
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title: "성능 최적화(Reflow & Repaint)"
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category: 10_Wiki/Topics
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status: verified
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canonical_id: self
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aliases: [Reflow, Repaint, Layout Thrash, 레이아웃 트래싱]
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duplicate_of: none
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source_trust_level: A
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confidence_score: 0.93
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verification_status: applied
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tags: [frontend, performance, browser, rendering, dom]
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raw_sources: []
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last_reinforced: 2026-05-10
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github_commit: pending
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tech_stack:
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language: javascript
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framework: browser
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---
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# 성능 최적화(Reflow & Repaint)
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## 매 한 줄
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> **"매 layout(=reflow) 의 비싸고, paint 의 cheap-er, composite 의 cheapest"**. 매 DOM/CSS write 의 invalidate 의 trigger — geometry change → reflow + repaint, color-only change → repaint, transform/opacity → composite-only. Layout thrashing 의 batch read/write 의 fix.
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## 매 핵심
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### 매 cost ladder
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1. **Composite only** (cheap, 60fps): `transform`, `opacity`, `filter` (some).
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2. **Paint** (medium): `color`, `background-color`, `box-shadow`, `border-radius`.
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3. **Layout (Reflow)** (expensive): `width`, `height`, `top/left`, `margin`, `padding`, `font-size`, `display`.
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4. **Full reflow trigger**: `<html>` font change, viewport resize.
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### 매 reflow triggers (write)
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- DOM 의 add/remove.
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- `display`/`position` change.
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- size/box-model property change.
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- font/text content change.
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- pseudo-class (`:hover`) 의 layout-affecting style.
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### 매 forced sync layout (read)
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- `offsetTop/Left/Width/Height`, `clientTop/...`, `scrollTop/...`.
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- `getComputedStyle()`, `getBoundingClientRect()`.
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- 매 pending invalidation 의 있을 때 read → 매 browser 의 immediate layout 의 trigger.
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## 💻 패턴
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### Layout thrash 의 fix (read-then-write)
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```js
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// X — N reflow (each iteration forces layout)
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boxes.forEach(b => {
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const w = b.offsetWidth;
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b.style.width = (w + 10) + 'px';
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});
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// O — 1 reflow (read all, then write all)
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const widths = boxes.map(b => b.offsetWidth);
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boxes.forEach((b, i) => b.style.width = (widths[i] + 10) + 'px');
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```
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### `requestAnimationFrame` batching
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```js
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let pending = false;
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function update(el) {
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if (pending) return;
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pending = true;
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requestAnimationFrame(() => {
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el.style.transform = `translateX(${nextX}px)`;
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pending = false;
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});
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}
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window.addEventListener('scroll', () => update(stickyEl));
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```
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### Detach → mutate → reattach (huge DOM ops)
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```js
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const list = document.getElementById('list');
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const fragment = document.createDocumentFragment();
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for (const item of items) {
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const li = document.createElement('li');
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li.textContent = item.name;
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fragment.appendChild(li);
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}
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list.appendChild(fragment); // 1 reflow, not N
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```
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### `transform` 의 substitute (animation)
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```css
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/* X — top change → reflow per frame */
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@keyframes slide-bad { from { top: 0 } to { top: 100px } }
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/* O — transform → composite only */
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@keyframes slide-good { from { transform: translateY(0) } to { transform: translateY(100px) } }
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```
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### `will-change` (sparingly)
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```css
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/* 매 animation 직전 의 hint, 매 finish 후 remove */
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.card { will-change: transform; }
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.card.done { will-change: auto; }
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```
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### `contain` (CSS containment)
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```css
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/* 매 subtree 의 layout/paint 의 isolate — outer 의 invalidation 의 not propagate */
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.widget { contain: layout paint; }
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```
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### Position fixed/sticky (composite layer)
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```css
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.toolbar {
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position: sticky; top: 0;
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/* GPU layer — scroll 시 reflow 없음 */
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}
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```
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### IntersectionObserver (no scroll handler)
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```js
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const io = new IntersectionObserver((entries) => {
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for (const e of entries) {
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if (e.isIntersecting) e.target.classList.add('in-view');
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}
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}, { rootMargin: '0px 0px -10% 0px' });
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images.forEach(img => io.observe(img));
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```
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## 매 결정 기준
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| 변경 | Cost |
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|---|---|
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| `transform` / `opacity` | composite only — 60fps OK |
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| `background-color` | paint — usually OK |
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| `top`/`left`/`width`/`height` | layout — animation 의 avoid |
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| `display` | layout — modal toggle 의 OK, animation 의 X |
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| `font-size` (root) | full document reflow — extreme cost |
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**기본값**: 매 animation → `transform/opacity` 만, batch read/write, large insert → DocumentFragment, off-screen → `content-visibility` / IntersectionObserver.
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## 🔗 Graph
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- 부모: [[브라우저 렌더링 파이프라인(Critical Rendering Path)]]
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- 응용: [[Virtual List]] · [[Animation Performance]]
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## 🤖 LLM 활용
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**언제**: layout thrash 의 detect (read-write interleave pattern), animation property 의 review.
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**언제 X**: actual paint profiling — Chrome DevTools Performance/Layers panel 의 use.
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## ❌ 안티패턴
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- **`will-change` everywhere**: 매 GPU memory exhaustion.
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- **`top/left` animation**: 매 reflow per frame — `transform` 의 substitute.
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- **scroll handler 의 layout read**: 매 thrash — IntersectionObserver 의 use.
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- **`innerHTML` 의 loop**: 매 N reflow — fragment 의 build.
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## 🧪 검증 / 중복
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- Verified (web.dev/articles/animations-guide, Paul Lewis "Avoid Large, Complex Layouts", CSSOM View spec).
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- 신뢰도 A.
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## 🕓 Changelog
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| 날짜 | 변경 |
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|---|---|
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| 2026-05-08 | Phase 1 |
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| 2026-05-10 | Manual cleanup — reflow trigger + 8 패턴 + 결정 기준 의 정리 |
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