JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div class="box">a</div>
    <div class="box">b</div>
    <div id="expandMe">expand me</div>
</div>

CSS

#container {
    border:1px solid red;   
    width:200px;
}

#container div {
    border: 1px solid black;
}

#container div.box {
    float:left;   
}

#expandMe {
    overflow: auto;
}