JSFiddle - React, Tailwind, and code Playground

HTML

<div class='fixed header'></div>
<div class='fixed side'></div>
<div class='scrollable'></div>

CSS

.scrollable {
    height: 2000px;
    width: 500%;
    margin: 0 auto;
    border: 1px dashed black;   
}

.fixed {
  position: fixed;
}

.header{
  top: 0;
  left: 0;
  right: 0;
  height: 100px;
  background-color: #000;
}

.side {
  top: 0;
  left: 0;
  bottom: 0;
  width: 50px;
  background-color: red;
}