Done in r22700.
I can no longer reproduce the visual corruption with the cairo backend, and the pixmap backend has been re-implemented by copying the whole pixmap buffer to a new backing before applying the copying, which seems to have fixed it. It is a little bit expensive to do all this copying, but most modern computers should have enough memory bandwidth for that.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2295