JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
  <div id="header">
    Header
  </div>
  <div id="middle">
    <div id="inner">
      This should be full height.
    </div>
  </div>
</div>

CSS

#outer {
  height: 100px;
  display: flex;
  flex-direction: column;
}

#header {
  height: auto;
}

#middle {
  flex-grow: 1;
  background-color: green;
  display: flex;
  flex-direction: column;
}

#inner {
  height: 100%;
  background-color: red;
  display: block;
  flex: 1;
}