JSFiddle - React, Tailwind, and code Playground

HTML

<div id="div1">
    <div id="div2"></div>
</div>

<div id="popup"></div>

<button> show popup</button>

CSS

div
{
     position: absolute;
    top: 0;
}

#div1 {
    width: 200px;
    height: 200px;
    background: red;
}
#div2 {
    width: 50px;
    height: 50px;
    background: blue;
    z-index: -1;
}

#popup
{
    display: none;
    
    width: 500px;
    height: 210px;
    opacity: .7;
    background: #333;
    
    z-index: 1;
}

button
{
    margin-top: 200px;
}

JavaScript

$('button').click(function()
{
    $('#popup').show();
    $('#div2').css('z-index', '2');
})