JSFiddle - React, Tailwind, and code Playground

HTML

<div id="blocker"></div>
<div id="contentcontain">contentcontain</div>

CSS

#blocker {
    position: absolute;
    top: 0px;
    left: 0px;
    z-index: 10;
    width: 100px;
    height: 70px;
    background: red;
    opacity: 0.8;
}

#contentcontain {
    position: relative;
    z-index: 5;
}

JavaScript

$(function() {
    $('#contentcontain').animate({ marginTop: 40 }, 1000, function() {
        $(this).css({ zIndex: 15 });
    });
});