JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
  <div class="header">
  </div>
  <div class="nav-wrapper">
    <div class="nav">
    </div>
  </div>
</div>

CSS

body {
  margin: 0;
}
.wrapper {
  position: relative;
}

.header {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  height: 50px;
  background-color: #373737;
}

.nav-wrapper {
  width: 400px;
  margin: 0 auto;
  position: relative;
}
.nav {
  position: absolute; 
  top:50px; 
  left: 0;
  width: calc(((100vw - 400px) / 2) + 400px);
  height: 50px;
  background-color: #474747;
  max-width: 100vw;
}