JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="fixo"></div>
</div>

CSS

body {
    margin:0px;
    padding:0px;
}
.container {
    background: #F00;
    width:100%;
    height:3000px;    
}

.fixo {
    width:100px;
    height:50px;
    background:#000;
    position:fixed;
}

JavaScript

var container = document.querySelector('.fixo'),
    w = window,
    d = document,
    e = d.documentElement,
    g = d.getElementsByTagName('body')[0],
    x = w.innerWidth || e.clientWidth || g.clientWidth,
    y = w.innerHeight|| e.clientHeight|| g.clientHeight;

container.style.top = (y / 2) - (container.clientHeight / 2) + 'px';
container.style.left = (x / 2) - (container.clientWidth / 2) + 'px';