- load the attached zul with Firefox
- click the content of a Groupbox instead of its header, drag it on another groupbox
the parent Vbox doesn't have a drop effect (z-drag-over)
if you drag a Groupbox by its header, it will trigger a drop effect.
drop effect appears on Vbox
- chrome doesn't have such bug
- 8.0.0, 8.0.1 don't have this bug.