JSFiddle - React, Tailwind, and code Playground
HTML
<div class="out">
<div class="a"></div>
<div class="b"></div>
</div>
CSS
.out{
background: red;
}
.a,.b{
width: 200px;
height:200px;
border: 1px solid black;
background: blue;
}
.a{position: fixed;
bottom:0}
.out:hover .a:not(:hover) + .b{
background: green;
}