JSFiddle - React, Tailwind, and code Playground

HTML

<div class="b">
    <button>Show when I hover</button>
</div>
<div class="A">When I hover over this the background should change</div>

CSS

.b {
    float: right;
    display: none;
}
.A {
    float: left;
    display: inline;
    width: 1000px;
}

.A:hover {
    background: gray;
}

.A:hover + .b {
    display: block;
}