JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div class="content">
        This is the content
    </div>
    <div class="sidebar">
        And this is the sidebar! I dynamically make myself bigger based on the content on the left!
    </div>
</div>

CSS

#container { position:relative; }
.content {background:#CCF; margin-right:150px; height:150px;}
.sidebar { background:#CFC; position:absolute; top:0; bottom:0; right:0; width:150px; }