JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="inner">
    <div class="full-height">
        Fix unscrollable content
    </div>
        <div style="width:400px;height:400px;background:black:color:white">Hello World</div>
       
    </div>
    </div>

CSS

* { box-sizing: border-box; }

.container {
    position: relative;
    border: solid 1px red;
    height: 256px;
    width: 256px;
    overflow: auto;
    
    float: left;
    margin-right: 16px;
}

.inner {
    position: relative;
    height: 100%;
}

.full-height {
    position: absolute;
  bottom:0;
    width: 100%;
    color:white;
    background: blue;
}