JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
    zx
</div>
<div class="fix">
    <button>Click</button>
</div>

CSS

html, body{
    height:100%;
    margin:0
}
.main{
    height:100%;
    width:50%;
    background:grey;
}
.fix{
    position:fixed;
    top:50%;
    border:red solid 2px;
    background:yellow;
    left:50%
}