JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
  <div id="header"></div>
  <div id="content"></div>
</div>

CSS

#parent {
  height: 200px;
}

#header {
  height: 40px;
}

#content {
  height: calc(100% - 40px); 
  overflow-y: auto
}

/* Not Important */

#header {
  background: green;
}

#content {
  background: red;
}