JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sidebar"></div>
<div class="maincontent">
  <div class="fixed--wrapper">
    <div class="fixed--header">
    
    </div>
  </div>
</div>

CSS

.sidebar {
  width: 20%;
  height: 100vh;
  overflow: hidden;
  position: fixed;
  background-color: #000;
  color: #fff;
  float: left;
}

.maincontent {
  float: right;
  position: relative;
  width: 80%;
  background-color: #D00;
  height: 300px;
}

.fixed--wrapper {
  width: 40%;
  position: relative;
  float: left;
  border-right: 1px solid rgba(0, 0, 0, 0.2);
  height: 100%;
  overflow-y: scroll;
  background-color: #E00;
}

.fixed--header {
  height: 60px;
  width: inherit;
  position: fixed;
  z-index: 2;
  background-color: #fff;
}