Fixes error when getting offset of element not in document

This fixes an error which is called out in jQuery Migrate but
probably never happens in real life. This is because we call
`jQuery.fn.offset` without checking if the element is in the
document. Based on testing done here and within the MediaWiki
team, I'm pretty sure jQuery never actually implemented explicit
checks and this jQuery Migrate warning is just to cover the case
where a browser might start returning inconsistnet results. And
we could at least reproduce the inconsistency, so that's something.

We now default the offset to 0/0 if the parent element happens to
not be in the document. This appears to be what jQuery used to do
in the past, and generally appears to be what people expect in
these cases.

This fixes #5584.

src/js/select2/dropdown/attachBody.js

@@ -190,7 +190,14 @@ define([
       $offsetParent = $offsetParent.offsetParent();
     }
 
-    var parentOffset = $offsetParent.offset();
+    var parentOffset = {
+      top: 0,
+      left: 0
+    };
+
+    if ($.contains(document.body, $offsetParent[0])) {
+      parentOffset = $offsetParent.offset();
+    }
 
     css.top -= parentOffset.top;
     css.left -= parentOffset.left;