Created attachment 1093649 [details]
Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1022241
When configure event is send to hidden popup window (it does Cinamon, Xfce and some other DE), the popup shown on wrong (old) position next time.
There's a testcase attached. It affects all popups created by Firefox - menus, dropdowns and so.
I think we had this fixed in gtk+ master with https://bugzilla.gnome.org/show_bug.cgi?id=758609 but then the issue re-appeared again later with https://git.gnome.org/browse/gtk+/commit/?id=67ab00e
should be fixed now