the 3 buttons are grey - I've tried F and shift F. I have a MAC and LR CC. I can't move the LR window--- it takes up the whole screen and won't shrink when I try to use the arrow at the corners. My only choice is to go into menu and choose window and minimize
Pressing F takes you to the new true full screen mode, where just the selected photo fills the screen. Press F again to go back to where you were. To cycle through the other screen modes you need to hold the Shift key and press F.
If that does not help please check the below link: