JSFiddle - React, Tailwind, and code Playground

HTML

<nav>
  Navigation
</nav>

<main>
  This should be square
</main>

CSS

* {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
  background-color: #fff;
}

nav {
  height: 100%;
  width: 96px;
  background-color: #666;
}

main {
  position: absolute;
  background-color: lightgreen;
  color: #fff;
  top: 0;
  left: 96px;
  height: calc(100vw - 96px);
  width: calc(100vw - 96px);
  max-width: 100vw;
}