We had identical problem in ZBX-2842 (fixed in 1.8.3)

This is identical but reproducible starting from 2.0.2, I mean 2.0.1 is ok in this regard.

This is caused (I've checked) by changes in ZBX-2876 (gui messaging should not work in popups).

As in 2.0 documentation is not specified that GUI messages don't work in frontend fullscreen mode -> so I consider this as a regression -> a bug -> should be fixed (IMO).
http://www.zabbix.com/documentation/2.0/manual/web_interface/user_profile/global_notifications