JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu">

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

CSS

#container { width: 100%; height: calc(100% - 30px); overflow: hidden; }
iframe { width: 100%; height: 100%; border: 0; }
html,body { margin: 0; height: 100%; }
.menu { height: 30px; background: red; }