JSFiddle - React, Tailwind, and code Playground

HTML

<div id="cont">
    <div id="numero">
        1        
    </div>
</div>
<script>
    document.getElementById('cont').onmousemove=moveDiv;
</script>

CSS

div#cont{
    position:relative;
    border-style: solid;
    border-width: 2px;
    width:500px;
    height:500px;
    margin-left:auto;
    margin-right:auto;
    padding:0;
}
div#numero{
    position:absolute;
    background-color:#ff3324;
    border-style: solid;
    border-width: 2px;
    width:30px;
    height:30px;
    font-family: "Lucida Grande", "Lucida Sans Unicode", "Lucida Sans", Geneva, Verdana, sans-serif;
    font-size:1em;
    line-height: 30px;
    text-align:center;
    margin-left:0;
    margin-right:0;
}

JavaScript

function moveDiv(e){
    if(!e){e=window.event;}
    var el = document.getElementById('numero');
    x = e.clientX-this.offsetLeft-this.clientLeft-el.offsetWidth/2;
    y = e.clientY-this.offsetTop-this.clientTop-el.offsetHeight/2;

    el.style.left = Math.min(Math.max(0,x),this.clientHeight-el.offsetWidth) +"px";
    el.style.top = Math.min(Math.max(0,y),this.clientHeight-el.offsetHeight) +"px";


}