JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div class="inner"></div>
    <div class="fixed"></div>
</div>

CSS

.parent {
    width: 300px;
    height: 300px;
    border: 1px solid black;
    overflow: auto;
    position: relative;
}
.inner {
    height: 1000px;
}
.fixed {
    width: 100px;
    height: 100px;
    background: lightblue;
    position: fixed;
    left: 0;
    bottom: 0;
}