Intersection Observer don't work with jQuery UI dialog

Question

This is related to Intersection observer does not work with target with position: fixed

but my issue is that interection observer don't fire on element with position: absolute. I have jQuery UI dialog and when it show up observer don't fire.

Here is my code:

var self = $('<div/>').appendTo('body').dialog({
    autoOpen: false
})
var first = true;
function visibility_checker(x) {
    if (first) {
        first = false; // ignore initial call
    } else {
        console.log(x);
    }
}
setTimeout(function() {
    self.dialog('open');
}, 4000);
if (window.IntersectionObserver) {
    var visibility_observer = new IntersectionObserver(visibility_checker, {
        root: document.body
    });
    visibility_observer.observe(self[0]);
}

<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<script src="https://code.jquery.com/ui/1.12.0/jquery-ui.min.js"></script>
<link href="https://code.jquery.com/ui/1.12.0/themes/smoothness/jquery-ui.css" rel="stylesheet"/>

Without jQuery UI

var self = $('<div class="x"/>').hide().appendTo('body');
var first = true;
function visibility_checker(x) {
    if (first) {
        first = false; // ignore initial call
    } else {
        console.log(x);
    }
}
setTimeout(function() {
    self.show();
}, 4000);
if (window.IntersectionObserver) {
    var visibility_observer = new IntersectionObserver(visibility_checker, {
        root: document.body
    });
    visibility_observer.observe(self[0]);
}

.x {
 width: 100px;
 height: 100px;
 background: red;
}

<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<script src="https://code.jquery.com/ui/1.12.0/jquery-ui.min.js"></script>
<link href="https://code.jquery.com/ui/1.12.0/themes/smoothness/jquery-ui.css" rel="stylesheet"/>

Anyone know what the issue is and why it don't fire?

Answer 1

Found the issue the problem is position: absolute, don't know why but setting:

body {
    position: relative;
}

fixed the issue, maybe absolute need to have reference. Also according to MDN you can use null as root. That also works.