In monitor settings at System settings, after re-ordering the screens correctly, if I want to disable the internal screen, even after disabling it the laptop screen will still be on and show Pantheon. Only after closing the lid does it disable it accordingly.
Also, after selecting secondary screen as primary, Pantheon did not move the main UI (upper panel, dock, etc) to that screen accordingly.
I'm using Intel HD Graphics 630 with Nvidia Optimus (NVIDIA GTX 1050) and NVIDIA'S proprietary drivers. I'm currently running with the NVIDIA card selected.
I haven't tested it with the Intel card selected, I'll get back to you on this. It'll probably have the same behaviour since the Intel card is the one managing the screens.