I am using LR CC 2015.3 Build 1052427.
It was set to 'Normal' and toggling it doesn't resolve the issue. However 'Fullscreen with Menubar' is an even better setting to my liking. Thanks for that.
Just to confirm are you on MAC OS X 10.11.2/10.11.1?
Windows controls were hidden by the menu-bar in 10.11.0. This bug was fixed with 10.11.1
Yep, running 10.11.2 (15C50)