Wow, that was fast! Thanks a lot, it works now.
There is a small ghost panel sprite on the screen though. This doesn't happen in demo room 12 or it's underneath one of the buttons, I'm not sure.
I investigated the camera problem a little further. stanncam disables application_surface and uses the Post-Draw event to draw everything. I added Update_GUI_dimensions() to a Post-Draw event in obj_GGL_controller and the menu is drawn properly. I still have to figure out how to center the items correctly on the screen though.