8:15:41jackdanielrandom-nick: this does the trick, thanks!
8:16:07jackdanielrain3: the patch from random-nick allows me to reproduce the issue locally, so I don't need to connect remotely to your machine - thanks for the opportunity though!
8:20:49jackdanieland having this reproduced I can quickly tell what's the underlying issue (fixing it is another matter) - indeed it is due to coordinate swizzling, but it is not a matter of malfunctioning computations
8:21:05jackdanielwe simply do not repaint the background when the native transformation is changed
8:22:40jackdanielthat said the fact that we do not redraw it is incorrect for more reasons, most noticeably because output is not idempotent (antialiasing is one, but there is also translucent ink)