JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="childA">
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
    aaaaaaaaaaaaaaa
  </div>
  <div class="childB">
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
    bbbbbbbbbbbbbbb
  </div>
</div>

CSS

.parent {
  display: block;
  position: relative;
  border: 1px solid black;
  height: 300px;
  width: 200px;
  
  display: flex;
  flex-flow: column;
}

.childA {
  display: block;
  background-color: grey;
  width: 100%;
  min-height:50%;
}

.childB {
  display: block;
  background-color: yellow;
  width: 100%;
  height: 100%;
  overflow: auto;
}