JSFiddle - React, Tailwind, and code Playground

HTML

<div id="buttons">
<div id="box"></div>
<div id="content"></div>
</div>

CSS

#box {
width: 10vw;
height: 10vw;
background-color: blue;
}

.box {
width: 10vw;
height: 10vw;
background-color: indigo;
float: left;
margin-left: 5px;
display: none;
}

#content {
width: 100%;
height: 10vw;
background-color: red;
padding: 5px;
}

JavaScript

$( "#box" ).on('mouseenter', function() {
 $( "#content").append("<div class='box'></div>");
 $('.box').fadeIn();
});