scrollStop.js

/*
  <div class="div1"></div>
	<div class="div2" id="Header"></div>
	<div class="div3" id="someDiv"></div>
*/

<script>
	window.addEventListener('scroll', function(ev) {

   		var someDiv = document.getElementById('someDiv');
   		var distanceToTop = someDiv.getBoundingClientRect().top;
   
    
		if(distanceToTop < 50){
			document.getElementById("Header").style.position = "fixed";
			document.getElementById("Header").style.top = "0";
			console.log("fixed")
		}else{
			document.getElementById("Header").style.position = "relative";
			console.log("relative")
		}
 
   		console.log(distanceToTop);
	});
</script>