JSFiddle - React, Tailwind, and code Playground

HTML

<div class="under">under</div>
<div class="over">over</div>

CSS

.under {
    position:fixed;
    top:0px;
    z-index:1;
    display:block;
    width:300px;
    height:200px;
    background-color:#aaa;
}

.over {
    display:block;
    width:300px;
    height:1200px;
    position:relative;
    z-index:2;
    margin-top:200px;
    background-color:green;
}