JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
<div class="scrollable"></div>
  <div class="fixed"><div class="fixedContent"></div></div>  
</div>

CSS

.container {
    width: 300px;
    height: 300px;
    overflow: auto;
}
.scrollable {
    height: 1000px;
    width: 100%;
    background: #eee;
}
.fixed {
    
    top: 0;
    left: 0;
    position: fixed;
}
.fixedContent {
    width: 50px;
    height: 50px;
    background: #f00;
    position: relative;
}