Suddenly, for some reason, I've lost the title bars on the windows for all applications.
I therefore can't activate a program window by clicking the title bar, because there's no title bar.
I also can't maximise or minimise a window, and I can't quit the program by clicking 'X' at the top right corner, because it's not there.
I don't know how this change has happened.
How can I get my title bars back?