JSFiddle - React, Tailwind, and code Playground

HTML

<div id="bla"></div>

CSS

#bla {
    width:400px;
    height:400px;
    background-color:green;
    display:none;
}

JavaScript

window.onmousemove=function(event){
    ev = event || window.event;
    if (event.pageX <= 400 && event.pageY <= 400){
        document.getElementById('bla').style.backgroundColor= "red";
    } else {
        document.getElementById('bla').style.backgroundColor= "green";
    }
}
setTimeout(function() {
         document.getElementById('bla').style.display="block";
},2000)