JSFiddle - React, Tailwind, and code Playground

HTML

<div id="top" class="shadow"></div>

CSS

body {
    height: 2000px; 
    background-color: white;
}
body:before,
#top,
body:after {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 80px;
    z-index: 9999;
    background-color: yellow;
}
body:before{
    content: '';
    display: block;
    z-index: 9997;
    -webkit-box-shadow: 0 1px 1px rgba(0, 0, 0, 0.3);
    -moz-box-shadow: 0 1px 1px rgba(0, 0, 0, 0.3);
    box-shadow: 0 1px 1px rgba(0, 0, 0, 0.3);
}
body:after {
    content: '';
    display: block;
    position: absolute; /* this needs to scroll */
    background-color: white; /* match background of body */
    top: 80px;
    height: 3px; /* covers height of :before shadow */ 
    z-index: 9998; /* overlap :before, moves on scroll */
}