A relatively light-weight "scroll spy" to pivot scrolling events from edge → level.

scroll-spy.js

{ // Detect scrolling of the viewport away from the absolute top of the document.
	function scrollHandler(e) {
		if ( window.scrollY == 0 && document.body.scrollTop == 0 ) document.body.classList.remove('offset')
		else document.body.classList.add('offset')
	}
	
	// Watch for the page being scrolled.
	window.addEventListener('scroll', scrollHandler)
	document.body.addEventListener('scroll', scrollHandler)
}