JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="placeholder"></div>
  <div class="child">
    some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  some long text <br />
  
  </div>  
</div>

CSS

.placeholder {
  height: 200vh;
  width: 50%;
  float: right;
}

.child {
  position: sticky;
  top: 0;
  bottom: 0;
  height: 100%;
  overflow-y: auto;
}