JSFiddle - React, Tailwind, and code Playground

HTML

<!-- http://jedicss.com/zadachka-vypadajushhee-menju-2/ -->

<ul class="menu">
    <li class="menu__item">
        <a href="https://goo.gl/9B3DHX">Задачка на прокачку</a>
        <ul class="menu menu-inner">
            <li class="menu__item"><a href="http://jedicss.com/">Jedicss.com</a></li>
            <li class="menu__item"><a href="https://goo.gl/r2Xcwn">Решение</a></li>
        </ul>
    </li>
</ul>

<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quis perspiciatis, at corporis sequi fugit minima consequatur dolores laboriosam unde illum libero temporibus, sit maxime.</p>
<p>Error illum iure praesentium ad est pariatur asperiores doloribus rem nesciunt rerum dolore, eaque! Esse cum, asperiores quas assumenda minima sapiente consequuntur soluta modi.</p>
<p>Harum voluptatum quidem, corrupti iusto! Sunt numquam molestias vitae officiis vel quas aspernatur, natus, aperiam ab dicta ad quasi officia accusantium, tenetur excepturi quaerat?</p>

CSS

.menu{
    margin: 0 auto;
    padding: 0;
    width: 180px;
    list-style: none;
}

.menu__item a{
    display: block;
    padding: 8px 16px;
    background-color: purple;
    color: #fff;
    text-decoration: none;
}

.menu__item a:hover{
    background-color: blue;
}

.menu-inner{
    border: 4px solid blue;
}

/*answer*/
.menu__item {
  position: relative;
}

.menu__item:hover::after {
  content: '';
  position: absolute;
  width: 100%;
  padding-top: 80px;
}

.menu__item:hover .menu-inner{
  visibility: visible;
  opacity: 1;
}

.menu-inner {
  position: absolute;
  visibility: hidden;
  margin-top: 80px;
  opacity: 0;
  transition: all .2s ease;
}

JavaScript

/* никакого js */