JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu">

</div>
<div id="container">
  <iframe...

CSS

#container { width: 100%; height: 100%; overflow: hidden; }
iframe { width: 100%; height: 100%; border: 0; }
html,body { margin: 0; height: 100%; }
.menu { height: 30px; background: red; }
body { height: 100%; display: flex; flex-direction: column; }
#container { flex: 1; }