JSFiddle - React, Tailwind, and code Playground

HTML

<div id="wrapper">
    <div id="left">
        left
    </div>
    <div id="right">
        right
    </div>
</div>

CSS

#wrapper {
    background-color: red;
    width: 200px;
    height: 100%;
    overflow: auto;
    padding: 5px;
    position:relative;
}

#left{
    width: 90px;
    float:left;
    background-color: #ccc;
    top:5px;
    left:5px;
    bottom:5px;
    position:absolute;
}

#right{
    width: 90px;
    height: 300px;
    float:right;
    background-color: blue;
}