JSFiddle - React, Tailwind, and code Playground

HTML

<div id="elm">
test
</div>

<div id="menu">
menu
</div>

CSS

#elm {
  width: 20px;
  height: 20px;
  background-color: blue;
  margin-bottom: 20px;
}

#menu {
  width: 20px;
  height: 20px;
  background-color: hotpink;
  display: none;
  opacity: 0;
  transition: opacity 1s linear;
}

JavaScript

var elm = document.getElementById('elm');
var menu = document.getElementById('menu');

elm.onmouseover = function () {
   menu.style.display = 'inline-block';
   setTimeout(function () {
       menu.style.opacity = 1;
   }, 1);
}