JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu">

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

CSS

#container { width: 100%; height: 100%; box-sizing: border-box; padding-top: 30px; overflow: hidden; }
iframe { width: 100%; height: 100%; border: 0; }
html,body { margin: 0; height: 100%; }
.menu { position: absolute; width: 100%; height: 30px; background: red; }