JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="navigation">
  <li>
    <a>drop down</a>
    <ul>
      <li>text</li>
      <li>text</li>
      <li>text</li>
      <li>text</li>
      <li>text</li>
      <li>text</li>
      <li>text</li>
      <li>text</li>
    </ul>
  </li>
</ul>

CSS

*{
  margin: 0;
  padding: 0;
  list-style: none;
}
#navigation{
  width: 100%;
  height: 60px;
  position:absolute;
  left: 0;
  top: 0;
  background:#225;
}
#navigation>li{
  width: 100px;
  height: 60px;
  float: left;
  margin-left:200px;
  text-align: center;
  line-height:60px;
  color:#EEE;
  cursor:pointer;
  /*to hold it's absolute positioned child*/
  position: relative;
}
#navigation>li:hover{
  background: rgba(250,250,250,0.1);
}

#navigation>li>ul{
  width: 150px;
  height: 250px;
  position: absolute;
  right: 0;
  top: 60px;
  background: #336;
  line-height: 80px;
  overflow-y: auto;
  /*transitions*/
  transition: all 0.3s ease;
  -moz-transition: all 0.3s ease;
  -webkit-transition: all 0.3s ease;
 
  /*to hide it by default*/
  opacity:0;
  pointer-events: none;
}
#navigation>li>ul>li{
  width:100%;
  height:50px;
  line-height:50px;
  float:left;
}

/*hovers*/
#navigation>li:hover>ul{
  opacity:1;
  pointer-events:auto;
}