Hi Camilla,
something must have gone wrong on your side if you still see images on your laptop screen? What you should see with our typical demos like PerceptualVBLSyncTest, LinesDemo, DotDemo etc., which all select screenId = max(Screen('Screens')); for Screen('Openwindow, screenId,...); is the normal desktop GUI with Matlab on the laptop display, and the PTB visuals on the HDMI projector or monitor.
Can you post the output of ResolutionTest in Matlab, and/or xrandr --screen 0 and xrandr --screen 1 in a terminal window? Also what’s the Matlab output of type /etc/X11/xorg.conf.d/90-ptbxorg.conf ?
You did run XOrgConfSelector and logged out / logged in again, right? The logout/login is when the freshly created config file is read and the display server configured accordingly, switching from single X-Screen to dual-X-Screen mode. The HDMI display must be plugged in at that time.
Oh and yes, you can disable the laptop panel from the command line, or with the display settings GUI of your desktop while the projector is plugged in, so this XOrgConfCreator/Selector/logout/login dance is not needed. The laptop will then switch between panel and HDMI projector, depending if HDMI is plugged in or not. This would increase performance slightly if needed. But often that is a bit impractical in a scanner setup, where experimenter laptop and scanner projector are in different rooms.
-mario