JSFiddle - React, Tailwind, and code Playground
HTML
<div class="another-parent">
<div class="overlay"></div>
</div>CSS
html, body, .another-parent {
position: relative;
height: 100%;
}
.overlay {
position: relative;
background-color: #39d;
width: 100%;
height: calc(100% - 123px);
}