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

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

/* 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

// 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

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)

/* 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

.widget {
  contain: layout paint;  /* 매 외부 영향 차단 — 매 reflow 매 widget 내부 만 */
}

requestAnimationFrame

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

content-visibility (skip rendering)

.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)
• 변형: 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