JSFiddle - React, Tailwind, and code Playground

HTML

<div style="height:100%;display:flex;flex-direction:column">
        <div style="background:#d0d0ff">
            This is a header
        </div>
        <div style="background:#d0ffd0;flex-grow:1">
            <!-- The new part -->
            <div id='newdiv' style="display:flex;flex-direction:row; height:100%">
                <div style="background:#ffd0d0"> Col 1 </div>
                <div> Col 2 </div>
            </div>
        </div>
    </div>

CSS

html, body {
   margin: 0;
   width: 100%;
   height: 100%;
}