JSFiddle - React, Tailwind, and code Playground

HTML

<body>


<div id="button" onmouseover="showMenu()" onmouseout="hideMenu()"></div>
<div id="menu"></div>
</body>

CSS

body{
  background-color:red;
}
#button {
    background-color: rgb(0,0,100);
    width: 100px;
    height: 50px;
    margin-top: 50px;
    margin-left: 50px;
}

#menu {
    background-color: rgb(0,100,0);
    width: 200px;
    height: 100px;
    margin-left: 50px;
    display: none;
}

JavaScript

function showMenu() {
    document.getElementById("menu").style.display = "block";
}

function hideMenu() {
    document.getElementById("menu").style.display = "none";
}