JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="tester"></div>
</div>
<span></span>

CSS

body, html {
    height: 100%;
}

#wrapper {
    height: 400%;
    background: #f44 ;
}

#tester {
    position: absolute;
    top: 150%;
    right: 0;
    width: 300px;
    height: 200px;
    background: #fff;
    border: 5px solid #000;
}

span {
  position: fixed;
  top: 5%;
  left: 5%;
}

JavaScript

var div = document.getElementById("tester"),
    info = document.querySelector("span"); // Juste pour voir

window.addEventListener("scroll", function(){
  animer = (window.pageYOffset || document.documentElement.scrollTop) + document.documentElement.clientHeight > div.offsetTop;
  info.innerHTML = animer; // Juste pour voir
}, false);