JSFiddle - React, Tailwind, and code Playground

HTML

<div class="dashboard">
    <div class="side panel">
        <div class="scroll">
            <div style="height: 400px">
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            </div>
        </div>
    </div>
    <div class="main panel">
        <div style="height: 400px">
        test
        </div>
    </div>
</div>

SCSS

html, body, .dashboard {
    position: absolute;
    display: block;
    top: 0; bottom: 0; height: 100%;
    left: 0; right: 0;  width: 100%;
}

html { font-size: 87.5% }

body {
    font-size:  1em;
    line-height: 1.5em;
    font-family: "Proxima-Nova-Regular", "ProximaNova-Regular", "Proxima Nova Regular", "ProximaNova Regular", "Proxima Nova", "Proxima Sans", "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif;
    text-rendering: optimizeLegibility;
    -webkit-font-smoothing: subpixel-antialiased;
       -moz-font-smoothing: subpixel-antialiased;
        -ms-font-smoothing: subpixel-antialiased;
         -o-font-smoothing: subpixel-antialiased;
            font-smoothing: subpixel-antialiased;
    background: #262626;
    color: #fff;
}

.dashboard {
    display: table;
}

.panel {
    display: table-cell;
    vertical-align: top;
    
    .scroll {
        display: block;
        position: relative;
        top: 0; bottom: 0; height: 100%;
        left: 0; right: 0;  width: 100%;
        overflow: auto;
    }
    
    &.side {
        width: 200px;
    }
}

* {
    outline: 1px solid red;
    outline-offset: -1px;
}