JSFiddle - React, Tailwind, and code Playground

HTML

<div class="viewportDiv">
  <div class="header">Site header</div>
  <div class="remainingDiv">
   <div class="mainSite">
   
   
    <div class="leftNav">
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      <div>item</div>
      
    </div>
    <div class="content">
      Lorem ipsum dolor sit amet, <br/> consectetur adipiscing elit. <br/>Aliquam vel ullamcorper ligula. Vestibulum nibh arcu, mollis in commodo ut, tristique sed sapien. Cras elementum facilisis consequat. Suspendisse eleifend gravida arcu, a rhoncus mi placerat at. <br/>Cras et vestibulum nulla, at vestibulum arcu. Pellentesque at aliquam turpis. Integer consequat tristique diam scelerisque consequat. <br/>Nullam mattis condimentum diam, sed tincidunt lacus iaculis a. Sed eu ex arcu. <br/>Nullam libero enim, venenatis quis vehicula id, fermentum vitae sapien. Praesent vehicula est id orci rhoncus, <br/>dignissim sodales risus commodo.
      Lorem ipsum dolor sit amet, <br/> consectetur adipiscing elit. <br/>Aliquam vel ullamcorper ligula. Vestibulum nibh arcu, mollis in commodo ut, tristique sed sapien. Cras elementum facilisis consequat. Suspendisse eleifend gravida arcu, a rhoncus mi placerat at. <br/>Cras et vestibulum nulla, at vestibulum arcu. Pellentesque at aliquam turpis. Integer consequat tristique diam scelerisque consequat. <br/>Nullam mattis condimentum diam, sed tincidunt lacus iaculis a. Sed eu ex arcu. <br/>Nullam libero enim, venenatis quis vehicula id, fermentum vitae sapien. Praesent vehicula est id orci...

CSS

body {
    height: 100%;
    margin: 0;
}

.viewportDiv {
  display: flex;
  flex-flow: column;
  height: 100%;
}

.header{
  background-color: yellow;
  height: 100px;
  min-height: 100px;
}

.remainingDiv{
  background-color: red;
  flex-grow: 1;
}

.mainSite {
  display: flex;
  flex-direction: row;
  flex-grow: 1;
  
  /* max-height: 100px; */
  
}

.leftNav{
  background-color: green;
  width: 200px;
  overflow-y: scroll;
}

.content {
  background-color: grey;
  overflow-y: scroll;
}