Eliminating Header Flicker in React: Using the IntersectionObserver API

July 20, 2024

Solve sticky header flicker issues by using IntersectionObserver and a sentinel element. This approach ensures smooth transitions and better performance compared to using window.scrollY. Learn the implementation details and improve your React app's user experience with our guide and code samples.

When building modern web applications, creating a smooth user experience is essential. A common challenge is managing the appearance and behavior of a sticky header as the user scrolls. In this post, we will explore why using the IntersectionObserver API with a sentinel element is a more robust solution for detecting scroll position changes compared to relying on window.scrollY.

The Problem: Flickering Header

Initially, we tried to implement a sticky header that changes its style based on the scroll position. The goal was to shrink the header when the user scrolls down and expand it when the user scrolls back up. Our initial approach used window.scrollY in a scroll event listener. However, this caused a flickering issue when the scroll position toggled between values close to our threshold.

❌ Initial Approach

import { useState, useEffect } from 'react';

export const Header = () => {
  const [isScrolled, setIsScrolled] = useState<boolean>(false);

  useEffect(() => {
    const handleScroll = () => {
      if (window.scrollY >= 50) {
        setIsScrolled(true);
      } else {
        setIsScrolled(false);
      }
    };

    window.addEventListener('scroll', handleScroll);

    return () => {
      window.removeEventListener('scroll', handleScroll);
    };
  }, []);

  return (
    <header className={isScrolled ? 'header-scrolled' : 'header'}>
      {/* Header content */}
    </header>
  );
};

While this method works in principle, it suffers from a significant drawback: frequent state toggling when the scroll position is near the threshold (e.g., between 49 and 51). This rapid toggling causes the header to flicker, degrading the user experience.

The Solution: IntersectionObserver with Sentinel Element

To solve the flickering issue, we use the IntersectionObserver API combined with a sentinel element. This approach is more robust and efficient for several reasons:

Precision: IntersectionObserver precisely detects when an element enters or leaves the viewport, avoiding the jitter seen with window.scrollY.
Performance: IntersectionObserver is designed for performance, offloading work to the browser's rendering engine and reducing the need for frequent state updates.
Buffer Zone: By using a sentinel element, we can create a natural buffer zone that smoothens the transition.

✅ Implementing the Solution

Here's how we can implement this using a sentinel element and the IntersectionObserver API:

import { useState, useEffect, useRef } from 'react';

export const Header = () => {
  const [isScrolled, setIsScrolled] = useState<boolean>(false);
  const sentinelRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        setIsScrolled(!entry.isIntersecting);
      },
      { threshold: [0] }
    );

    if (sentinelRef.current) {
      observer.observe(sentinelRef.current);
    }

    return () => {
      if (sentinelRef.current) {
        observer.unobserve(sentinelRef.current);
      }
    };
  }, []);

  return (
    <>
      <header className={isScrolled ? 'header-scrolled' : 'header'}>
        {/* Header content */}
      </header>
      <div ref={sentinelRef} style={{ height: '1px' }} />
    </>
  );
};

Why This Works

Stability: The IntersectionObserver detects visibility changes of the sentinel element, providing a stable trigger point for state changes.
Smooth Transitions: The sentinel element's presence ensures that the header only changes its state when it is completely in or out of view, avoiding rapid state toggling.
Performance Efficiency: Offloading the scroll detection to the browser's optimized IntersectionObserver improves performance, especially on scroll-heavy pages.

Conclusion

By using the IntersectionObserver API with a sentinel element, we can create a smooth and stable sticky header experience. This method overcomes the limitations of relying on window.scrollY for scroll detection, providing a more robust solution for managing dynamic header styles.

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