I was just moving some stuff around and somehow I spun my border and I don't know how to get it back. Here's a picture. When is spin the picture the border just stays there.

There is a canvas rotation widget in the lower corner of the window:

Thanks!

ctrl+middle-mouse-button drag - no way to turn it off. At the moment you notice the canvas is rotating add shift key and roll back - or reset at the widget Tyler mentioned.