"From my tests the size of the document is irrelevant, as is the zoom level, resolution in fact everything, but that's not to say this will be the situation for everyone."
Odd. Doc size is the key for me. Resolution seems to make no difference. Zoom seems to 'fix' it, but only for particular zoom levels in particular circumstances. It's by no means a solution. Your solution involving moving the canvas* does seem to be a definite, if temporary, fix though - and that's fine for me. Thanks so much for discovering it!
Ahh, bugs!
[*edited for clarity]