JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <p>Some text</p>
</div>

<div class="float">
  <div class="sidebar">
    <p>Sidebar</p>
  </div>
</div>

CSS

.container {
  width: 100%;
  height: 600px;
  background-color: red;
  max-width: 400px;
  margin-left: auto;
  margin-right: auto;
}

.float {
  position: relative;
  height: 10px;
  width: 100%;
  max-width: 800px;
  margin-left: auto;
  margin-right: auto;
  background-color: blue;
}

.sidebar {
  position: fixed;
  top: 0;
}