JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
  <div id="something">
  div of varying height
  </div>
  <div id="scroller">
    <div id="child">
      meh
    </div>
  </div>
</div>

CSS

#parent {
  background-color: blue;
  padding: 10px;
  height: 500px;
  overflow: hidden;
  display: flex;
  flex-direction:column;
}

#scroller {
  overflow: auto;
  padding: 10px;
  background-color: red;
  flex-grow:1;
}

#child {
  height: 10000px;
  background-color: green;
}