SO does not use the plugin, it is built only for SO, they just create <div>and attach it to the page with the following style:
.popup {
background-color:#fff;
border:solid 10px #AE0000;
-webkit-box-shadow:2px 2px 5px #000;
-moz-box-shadow:2px 2px 5px #000;
box-shadow:2px 2px 5px #000;
z-index:1;
display:none;
position:absolute;
padding:15px;
}
They simply insert the created <div class="popup">wherever it is on the page, in which case it is added to the parent element of the clicked link closed-question-nnnnnn( .post-menudiv).
, , , , , .