Resizer to expand and minimize div

You can start by doing your own minimal example like the below one that you can improve. I think its better than trying to reproduce a complex code like the codepen one:

It works on the hover but you can easily make it on click with some JS

.container {
  display: flex;
  height: 200px;
}

.container>div {
  flex: 1;
  background: 
    linear-gradient(red, red) left/15px 100% no-repeat, 
    blue;
  padding-left: 15px;
  color: #fff;
  transition: 0.5s all;
}

.container:hover>div {
  min-width: 15px;
  padding: 0;
  flex: 0;
  writing-mode: vertical-lr;
}

.container>div:hover {
  flex: 1;
  padding-left: 15px;
  writing-mode: initial;
}
<div class="container">
  <div>HTML</div>
  <div>CSS</div>
  <div>JS</div>
</div>