Scroll behavior control in NextJS - Performance & Optimization

Performance: Scroll behavior control

MEDIUM IMPACT

Controls how the page scrolls and affects user interaction smoothness and visual stability during navigation.

Implementing smooth scrolling on navigation or user actions

NextJS

window.scrollTo({ top: 0, behavior: 'smooth' });

Uses native smooth scrolling without layout changes, reducing reflows and visual shifts.

📈 Performance GainSingle repaint, no reflows, minimal CLS impact.

Implementing smooth scrolling on navigation or user actions

NextJS

window.scrollTo({ top: 0, behavior: 'auto' });
document.body.style.overflow = 'hidden';
setTimeout(() => { document.body.style.overflow = 'auto'; }, 500);

Forces instant scroll jumps and toggles overflow causing layout reflows and visual shifts.

📉 Performance CostTriggers 2 reflows and causes noticeable CLS during overflow toggle.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Instant scroll with overflow toggle	Multiple style changes	2 reflows	High paint cost due to layout shifts	[X] Bad
Native smooth scroll API	No extra DOM changes	0 reflows	Low paint cost with smooth compositing	[OK] Good

Rendering Pipeline

Scroll behavior affects the Layout and Paint stages by triggering reflows or repaints depending on how scrolling is handled.

→Layout

→Paint

→Composite

⚠️ BottleneckLayout when overflow or scroll position changes cause reflows

Core Web Vital Affected

INP, CLS

Controls how the page scrolls and affects user interaction smoothness and visual stability during navigation.

Optimization Tips

1Prefer native smooth scroll APIs over manual scroll position changes.

2Avoid style changes like overflow toggling during scroll to prevent layout reflows.

3Test scroll performance using browser DevTools Performance panel to catch layout thrashing.

Performance Quiz - 3 Questions

Which scroll behavior method causes fewer layout reflows?

A window.scrollTo with behavior 'smooth'

B window.scrollTo with behavior 'auto' plus style overflow toggling

C Manually changing scrollTop in a loop

D Using CSS scroll-behavior: auto

DevTools: Performance

How to check: Record a session while triggering scroll behavior. Look for Layout and Paint events during scroll.

What to look for: Minimal Layout events and smooth frame times indicate good scroll performance.