1.) It seems that disabling the battlescape HUD is just possible after a change of the height level (Cursor up or down), otherwise hitting the F11 key shows no effect @ all.
2.) When pressing ESC afterwards to get the HUD back, the battlescape is now black & disappeared, but you can bring it back by hitting ESC twice (Back-To-Menu/Back-To-Game)
3.) In high resolutions (1920x1200 for example) not the full screen is used when you hide the HUD.
Bug Tracker item here:
https://sourceforge.net/tracker/?func=detail&aid=3179698&group_id=157793&atid=805242
