JSFiddle - React, Tailwind, and code Playground

HTML

<div id="big-container">


<div id="bottom-menu">
<span>Some Content</span>
</div>
</div>

CSS

#big-container {
  height: 5000px;
  width:5000px;
  background:#000;
  position:relative;
}

#bottom-menu {
  position:fixed;
  bottom:0;
  left:50%;
  width:250px;
  margin-left:-125px;
  //optional
  padding: 10px;
  height:50px;
  line-height:50px;
}
#bottom-menu span {
  display:inline-block;
  width:100%;
  text-align:center;
  background-color:#fff;
}