JSFiddle - React, Tailwind, and code Playground

HTML

<nav></nav><div></div>

CSS

nav,div {display: inline-block;vertical-align: top;}

nav {width: 30%;height: 2000px;background: #333;}
div {width: 70%;height: 1500px;background: #444;}

JavaScript

var el = document.querySelector("div");

window.onscroll = function(){

  el.style.backgroundColor =  (Math.abs(el.getBoundingClientRect().top) >= el.offsetHeight - window.innerHeight) ? "#09f" : "#444";
  	

}