This is one we're aware of indeed. If the canvas is wide aspect, or is started in a situation where it is about as big as the ArtRage window, then rotated to say 45 degrees or something like that, bits of the canvas will be protruding outside of the Window. Your best best at this point would be to hold down the spacebar and drag the canvas into a more accessible location. You can reset the rotation and position from the top menu if you want to later.
Hope that's of some help.