JSFiddle - React, Tailwind, and code Playground

HTML

<div class="left-column">left</div>
<div class="top-bar">top</div>

CSS

.page-wrapper, html, body {
    width:100%;
    height:100%;
    margin:0;
    padding:0;
}

.left-column {
    float:left;
    width:150px;
    height:100px;
    background:#090909;
    color:white;
}

.top-bar {
    margin-left:150px;
    background:yellow;
    border:2px solid red;
}