JSFiddle - React, Tailwind, and code Playground

HTML

<div class="fixed">
    <div class="center"></div>
</div>

CSS

body {
    height: 4000px;
}

.fixed {
    position: fixed;
    width: 100%;
    height: 40px;
    background: tomato;
}

.center {
    width: 300px;
    margin: auto;
    height: 40px;
    background: blue;
}