JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div></div>
</div>

CSS

/* The rules in this selector are not part of the problem - they're part of the solution. */
/* (They allow us to actually see the problem) */
#container
{
    /* Needed to see problem */
    position: relative;
    color: #fff;
    width: 100px;
    height: 100px;
    background: #000;
    margin-top: 80px;
}

/* The rules in this selector are not part of the problem - see above: */
#container *
{
    background: #222;
    position: absolute;
    opacity: 0;
    width: 50px;
    height: 5px;
}

/* Here starts the actual problem code - change any one of these, and the border
   should disappear - except they're all rather integral to the code. */

#container
{
    /* POSSIBLY contributes to problem - sometimes dropping this fixes it, sometimes not */
    display: inline-block;
}

#container * {
    /* Contribute to problem */
    border-radius: 5px;
    transition: all .3s; /* Including -webkit-transition */

    /* POSSIBLY contributes to problem - sometimes dropping this fixes it, sometimes not */
    padding: 5px;
}

#container:hover *
{
    /* Contribute to problem */
    bottom: calc(100% + 10px);
    opacity: 0.9;
}