JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="content">hover over sidebar</div>
  <div class="sidebar">
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
    <img class="icon" src="http://cdn.onlinewebfonts.com/svg/img_98883.svg" />
  </div>
</div>

CSS

.container {
  width: 400px;
  height: 200px; 
  background-color: grey;
  display: flex;
  flex-direction: row;
}

.content {
  width: 100%;
  margin: 20px;
  border: 1px solid red;
  margin: 10px;
}

.sidebar {
  background-color: lightgrey;
  display: flex;
  flex-flow: column wrap-reverse;
  overflow: hidden;
}

.sidebar:hover { overflow: visible; width: 75px; }

.icon {
  width: 20px;
  height: 20px;
  padding: 5px;
}

.icon path { fill: white }