JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
		<div class="sidebar">sidebar</div>
		<div class="content">content</div>
	</div>

CSS

.wrapper {
  max-width: 300px;
  margin: 0 auto;
  background-color: green;
  position: relative;
}
.wrapper .content {
  height: 2000px;
}
.wrapper .sidebar {
  width: 100px;
  background-color: red;
  position: fixed;
  top: 0;
  left: 0;
  height: 100%;
}