JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mask"></div>
<div id="box"></div>
<textarea rows="2">When a scrollbar is needed the handle on the textarea becomes visible</textarea>

CSS

#mask{
    position:absolute;
    left:100px;
    top:0px;
    width: 100px;
    height:60px;
    background-color: white;
    border:thin black solid;
}
#box{
    position:absolute;
    left:158px;
    top:28px;
    width: 20px;
    height:20px;
    border:thin black solid;
}
textarea{
    position: absolute;
    left:10px;
    top:10px;
    z-index:-1;
}