JSFiddle - React, Tailwind, and code Playground

HTML

<div id="expander">
    Expander
    <div id="thumb">Thumbnail</div>
</div>

CSS

#expander {
  position: absolute;
  top: 100px;
  left: 100px;
  width: 200px;
  height: 100px;
  background: #ff0;
}

#thumb {
  position: absolute;
  top: -50px;
  right: -100px;
  width: 200px;
  height: 200px;
  opacity: 0.5;
  background: #00f;
  color: #fff;
}

JavaScript

var $expander = $('#expander');

$expander.hover(
    function() { $expander.animate({width: '400px'}); },
    function() { $expander.animate({width: '200px'}); }
);