JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">
    <div class="top"></div>
    <div class="bottom">
        <div class="inside"></div>
    </div>
</div>

CSS

body {
    margin: 0;
    padding: 0;
}

.header {
    position: relative;
    z-index: 1;
}

.top {
    background: aqua;
    height: 40px;
    opacity: 0.5;
    position: fixed; /* important – do not change! */
    width: 100%;
    z-index: 5; /* important – do not change! */
}

.bottom {
    background: green;
    height: 40px;
    overflow: hidden; /* important – do not change! */
    position: fixed; /* important – do not change! */
    top: 40px;
    width: 100%;
    z-index: 3; /* important – do not change! */
}

.inside {
    background: red;
    height: 40px;
    position: fixed; /* important – do not change! */
    top: 20px;
    width: 100px;
    z-index: 10; /* doesn't work! */
}