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HTML

              
                header.header
  .header-txt TITLE
main.main
  .main-top 1
  .main-middle 2
  .main-bottom 3
              
            
!

CSS

              
                header
  height 70px
  line-height 70px
  background rgba(255,255,255,.45)
  padding 0 20px
  position fixed
  top 0
  left 0
  width 100%
  transition background .45s
  &.scrolled
    background rgba(0,0,0,1)
    color #fff

.main
  text-align center
  font-size 2rem
  &-top
    height 250px
    line-height 250px
    background #d0d0d0
  &-middle
    height 350px
    padding-top 70px
    background #999
  &-bottom
    height 1500px
    padding-top 70px
    background #f5f5f5
              
            
!

JS

              
                (() => {
	const clientHeight = document.documentElement.clientHeight;
	const header = document.querySelector('.header');
	let observer = new IntersectionObserver((changes) => {
		for (const change of changes) {
			let rect = change.target.getBoundingClientRect();
			let h = (0 < rect.top && rect.top < clientHeight)	// The top of the object is in the display area.
			|| (0 < rect.bottom && rect.bottom < clientHeight)	// The bottom of the object is in the display area.
			|| (0 > rect.top && rect.bottom > clientHeight);	// The top end bottom end of the target is also not displayed, but the middle is displayed.
			if(h) {
										header.classList.add('scrolled');
			} else {
header.classList.remove('scrolled');
			}
		}
	});
	let target = document.querySelector('.main-bottom');
	observer.observe(target);
})();
              
            
!
999px

Console