JSFiddle - React, Tailwind, and code Playground

HTML

<div id='sidepane'>
</div>

<div id='background'>

 <div id='popup'>
 </div>

</div>

CSS

#sidepane {
  position: absolute;
  background-color: red;
  width: 200px;
  height: 200px;
  z-index: 2;
}

#background {
  position: absolute;
  background-color: green;
  width: 100%;
  height: 100%;
  z-index: 1;
}

#popup {
  position: absolute;
  background-color: blue;
  z-index: 3;
  left: 20px;
  top: 20px;
  width: 220px;
  height: 40px;
}