Older versions of Illustrator support Portrait Mode - it seems like CC does not. This is very useful if your main computer is a Microsoft Surface docked in portrait mode or have a secondary monitor in portrait mode, or if using a Surface will on the go you prefer to work in portrait mode. It seems like it would be simple to change as when the Illustrator window is not active it reverts to portrait mode anyway, also my old version (CS4) works just fine in this mode so it has been done before. I've attached a video showing how it works currently and why it is frustrating.