I found that sometimes arrow keys don't work because other windows steal focus. For example a docked toolbar may steal focus (even though it's docked!), and the zoom value field in the bottom left corner also loves to grab the focus and not let it go. This is an old problem (see "usability" programming task on Sodipodi site). I will see what I can do about it - but I'm afraid it's too complex for me, so if a real programmer could look into that, that would be nice. Basically, we need non-canvas windows to send focus and keystrokes to the (current) canvas whenever possible.

Still, if you click on the canvas first, the arrow keys always work :)