JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
  <div class="panel"></div>
  <div class="nav"></div>
</div>

CSS

*{
  margin: 0;
  border: 0;
  padding: 0;
  box-sizing: border-box;
}

html, body{
  height: 100%;
  min-height: 100%;
}

#main{
  display: flex;
  height: 100%;
}

.panel, .nav{
  padding: 15px;
  height: 100%;
  min-height: 100%;
  overflow: auto;
}

.panel{
  background: pink;
  width: 225px;
}

.nav{
  background: red;
  flex: 1;
}