As noted in#30490 (comment) (and#30485, the original issue, also#25075), Widget.canvas, which is typically initialized asself.useblit = useblit and canvas.supports_blit, is fragile against changes in the underlying canvas (the most common case in practice being a temporarily switch from a canvas that supports blitting to one that doesn't, when saving to vector output).
@tacaswell wrote some code to handle that in#25085, but some further fixes were necessary in#30490 which uses a different approach; a unified way of handling the issue would be nice to have.