I have a notebook that I connect to my school beamer with a HDMI cable.

In Trinity Control Center I can see both my laptop screen and the HDMI screen. 
I cannot use it if I don't "extend the desktop" to it, but then all my apps 
open stretched overt the two and control is very difficult.

I can set the external screen to the same resolution as the internal, but 
there is no option to just mirror the internal screen on the external.

Am I missing some black magic?

Thierry