JSFiddle - React, Tailwind, and code Playground

HTML

<div id="overlay" class="overlay">
  <form>
    foo
    <select>
        <option>a</option>
        <option>c</option>
        <option>d</option>
    </select>
  </form>
</div>
<div class="menu">
    <ul>
        <li id="a">aaaaa</li>
        <li>bbbb</li>
        <li>cccc</li>
    </ul>
</div>

CSS

.overlay {
    position: absolute;
    z-index: 2;
    left: 33%;
    display: none;
    width: 63%;
    height: 75%;
    border: 1px red solid;
    background-color: white;
}

.menu {
    float: left;
    width: 5%;
    z-index: 1;
}

JavaScript

$(function() {
    $('#a').mouseenter(function() {
        $('#overlay').show();
    });
    $('#overlay').mouseleave(function() {
        $(this).hide();
    });
    $('#overlay select').mouseleave(function(ev) {
        ev.stopPropagation()
    });
});