Thanks for the clarification. I don't own a laptop, but I help people who use
them. I've never seen a laptop which was running Ubuntu, so my comment was about
what I have seen, laptops running Windows, always showing the same thing on
external and internal displays. Turns out my experience is incomplete.

Last month I set up a laptop where it would have been handy to put the
multi-location meeting video on the projector while running Skype on the
internal screen. It never occurred to me that it might be possible, and I'm 99%
certain that it was not, on that particular laptop.