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.

Practice

1. What does setting behavior: 'smooth' in window.scrollTo do in a Next.js app?

easy

A. It reloads the page after scrolling.

B. It makes the page scroll smoothly instead of jumping instantly.

C. It scrolls the page to the top instantly.

D. It disables scrolling on the page.

Solution

1. Step 1: Understand the scroll behavior option

   The behavior property controls how scrolling happens. Setting it to 'smooth' means the scroll moves gradually.

2. Step 2: Apply to Next.js scrolling

   Using window.scrollTo({ behavior: 'smooth' }) in Next.js makes the page scroll smoothly instead of jumping instantly.

3. Final Answer:

   It makes the page scroll smoothly instead of jumping instantly. -> Option B

4. Quick Check:

   behavior: 'smooth' = smooth scrolling [OK]

Hint: Remember 'smooth' means gradual scroll, not instant jump [OK]

Common Mistakes:

• Thinking 'smooth' disables scrolling
• Confusing 'smooth' with instant scroll
• Assuming it reloads the page

2. Which of the following is the correct syntax to scroll smoothly to the top of the page in Next.js?

easy

A. window.scrollTo({ smooth: 'top' });

B. window.scrollTo(0, 0, 'smooth');

C. window.scrollTo({ top: 0, smooth: true });

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

Solution

1. Step 1: Review window.scrollTo syntax

   The correct syntax uses an object with top and behavior keys: { top: 0, behavior: 'smooth' }.

2. Step 2: Identify correct option

   window.scrollTo({ top: 0, behavior: 'smooth' }); matches the correct syntax. Others misuse parameters or keys.

3. Final Answer:

   window.scrollTo({ top: 0, behavior: 'smooth' }); -> Option D

4. Quick Check:

   Correct scrollTo syntax = window.scrollTo({ top: 0, behavior: 'smooth' }); [OK]

Hint: Use object with top and behavior keys for smooth scroll [OK]

Common Mistakes:

• Passing parameters as separate arguments instead of object
• Using incorrect keys like 'smooth' instead of 'behavior'
• Misplacing the 'smooth' value

3. Consider this Next.js code snippet inside a component:
<pre>useEffect(() => { router.events.on('routeChangeComplete', () => { window.scrollTo({ top: 0, behavior: 'smooth' }); }); }, []);
What will happen when the route changes?

medium

A. The page will smoothly scroll to the top after navigation.

B. Nothing will happen because the event listener is not removed.

C. The page will jump instantly to the top.

D. The page will reload after scrolling.

Solution

1. Step 1: Understand event listener on routeChangeComplete

   The code listens for the route change completion event and triggers scrolling.

2. Step 2: Analyze scrollTo call

   It calls window.scrollTo with behavior: 'smooth', so scrolling is smooth to top.

3. Final Answer:

   The page will smoothly scroll to the top after navigation. -> Option A

4. Quick Check:

   routeChangeComplete + smooth scroll = smooth scroll top [OK]

Hint: routeChangeComplete triggers smooth scroll to top [OK]

Common Mistakes:

• Assuming instant jump instead of smooth scroll
• Thinking event listener removal is required to trigger scroll
• Confusing scroll with page reload

4. This Next.js code tries to scroll smoothly on route change:
<pre>useEffect(() => { router.events.on('routeChangeComplete', () => { window.scrollTo({ top: 0, behavior: 'smooth' }); }); }, []);
But after navigating multiple times, scrolling stops working. What is the likely problem?

medium

A. The event listener is added multiple times without cleanup, causing issues.

B. The scrollTo syntax is incorrect and causes errors.

C. Next.js does not support scroll control on route changes.

D. The behavior 'smooth' is not supported in modern browsers.

Solution

1. Step 1: Identify event listener management

   The code adds an event listener on mount but never removes it, so multiple listeners accumulate.

2. Step 2: Understand effect of multiple listeners

   Multiple listeners cause unexpected behavior or conflicts, stopping scroll from working properly.

3. Final Answer:

   The event listener is added multiple times without cleanup, causing issues. -> Option A

4. Quick Check:

   Missing cleanup causes multiple listeners = problem [OK]

Hint: Always remove event listeners in useEffect cleanup [OK]

Common Mistakes:

• Ignoring cleanup of event listeners
• Blaming scrollTo syntax which is correct
• Assuming browser support is the issue

5. You want to scroll smoothly to a specific section with id 'contact' after navigating to a new page in Next.js. Which approach correctly achieves this?

hard

A. Use window.scrollTo(0, 0); after navigation and then call scrollIntoView without behavior option.

B. Use window.scrollTo({ top: document.getElementById('contact').offsetTop, behavior: 'auto' }); inside routeChangeStart event.

C. Use router.events.on('routeChangeComplete', () => { document.getElementById('contact').scrollIntoView({ behavior: 'smooth' }); });

D. Use router.push('/contact'); without any scroll control; rely on default browser behavior.

Solution

1. Step 1: Choose correct event and method

   Use routeChangeComplete to ensure page is loaded, then scroll to element with smooth behavior.

2. Step 2: Verify scrollIntoView usage

   scrollIntoView({ behavior: 'smooth' }) scrolls smoothly to the element with id 'contact'.

3. Final Answer:

   Use router.events.on('routeChangeComplete', () => { document.getElementById('contact').scrollIntoView({ behavior: 'smooth' }); }); -> Option C

4. Quick Check:

   routeChangeComplete + scrollIntoView smooth = Use router.events.on('routeChangeComplete', () => { document.getElementById('contact').scrollIntoView({ behavior: 'smooth' }); }); [OK]

Hint: Scroll after routeChangeComplete using scrollIntoView with smooth [OK]

Common Mistakes:

• Using routeChangeStart instead of routeChangeComplete
• Forgetting behavior: 'smooth' for smooth scroll
• Relying on default scroll without control