JSFiddle - React, Tailwind, and code Playground

HTML

<div id="panel">
    <div id="top">top<br /> panel</div>
    <div id="main">main</div>
</div>

CSS

html, body {
    height: 80%;
}
div {
    padding: 1em;
}
#panel {
    padding: 0px;
    height: 100%;
    position: relative;
    border: 3px solid red;
}
#top {
    background-color: steelblue;
}
#main {
    background-color: gray;
    margin-bottom: 0px;
}