2nd/10_Wiki/Topics/Frontend/레이아웃 스래싱(Layout Thrashing).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-레이아웃-스래싱-layout-thrashing	레이아웃 스래싱 (Layout Thrashing)	10_Wiki/Topics	verified	self	Layout Thrashing Forced Synchronous Layout FSL	none	A	0.9	applied	performance rendering javascript frontend		2026-05-10	pending	language framework javascript 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

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

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

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

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)

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)

.widget {
  contain: layout;  /* 매 widget 내부 변경 — 매 outer 매 reflow 의 X */
}

DevTools 측정

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)

// 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