JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="header">HEADER</div>
  <div class="body">
    <div class="leftArea">
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
      <p>left area</p>
    </div>
    <div class="rightArea">
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
      <p>right area</p>
    </div>
  </div>
</div>

CSS

.container {
  max-height: 30vh;
  border: 1px solid black;
  overflow-x: hidden;
  overflow-y: auto;
  /* new */ display: flex;
  /* new */ flex-direction: column;
}

.header {
  border: 1px solid red;
}

.body {
  display: flex;
  /* new */ min-height: 0;
}

.leftArea {
  border: 1px solid green;
  width: 50%;
}

.rightArea {
  border: 1px solid blue;
  width: 50%;
  overflow: auto;
}