JSFiddle - React, Tailwind, and code Playground

HTML

<div id="leftCol">
    <div id="left1">
        first leftCol extends with content<br />
        first leftCol extends with content<br />
        first leftCol extends with content
    </div>
    <div id="left2">
        second leftCol extends all the way down
    </div>
</div>
<div id="rightCol">
    rightCol<br />
    rightCol<br />    
    rightCol<br />
    rightCol<br />    
    rightCol<br />
    rightCol<br />    
</div>

CSS

#leftCol {
    float: left;
    width: auto;
}

#left1 {
    background-color: yellow;
}

#left2 {
    background-color: green;
        min-height: 100%;
    position: absolute
}

#rightCol {
        background-color: gray;
}