change div position as user scrolls

divChangeFix.html

<style>
#altContainer {
  position:absolute;
  min-height:500px;
  width:400px
  
  
  }
  
  #alternateLinks {
     position:absolute;
    height:200px;
    width:100%;
  }
<style>

<div id="altContainer">
  <div id="alternateLinks">
    
  </div>
  
</div>

<script>
   var container = document.getElementById("altContainer");
		var scroller = document.getElementById("alternateLinks");

		// Scroll function for repositioning 'scrollerr'.

		function scroll() {

			var cY = container.getBoundingClientRect().top;
			var cH = container.getBoundingClientRect().height;
			var sH = scroller.getBoundingClientRect().height;

			var top = 300;

			scroller.style.top = Math.min(Math.max(-cY + top, 0), cH - sH) + 'px';
		};

		// Init scrolling.

		window.addEventListener("scroll", scroll);
		scroll();
</script>