I created a new problem, and gave my work-around because the original thread is locked and I cannot add.
As for when it happens, I am not sure. I touched something, maybe F11, and it went into full screen. I assumed that Esc would get me out, it didn't. I had to 'Alt-Tab' and go to a different window and google in Firefox, and came across that forum entry above, with no solution. I killed Opera, relaunched, still stuck, and no 'Exit full screen' etc. option anywhere. I went into settings because it was possible and there I see the 'Exit... ' in context menu.
Just implement 'Esc' or add 'Exit Full Screen' in the context menu, please.
Yes indeed -- tablet mode forces your computer to work like an iPad, where everything is always full screen. Some people only use Windows if it acts like an iPad.