JSFiddle - React, Tailwind, and code Playground

HTML

<div class='container'>
  <div class='left'>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>left</p>
    <p>end</p>
  </div>
  <div class='right'>
    right
  </div>
</div>

CSS

.container{
    display: flex;
    flex-flow: row;
    align-items: stretch;
    height: 200px;
}

.left{
  flex: 0 1 100px;
  overflow: auto;
  background-color: red;
}

.right{
  flex: 1;
  background-color: green;
}