I don't recall seeing this before so I think it broke not too long ago. Make
a painting and scribble around so that you've got some good coverage of the
painting surface. Bring up the halos and use the bottom right one to resize
the are smaller so that it intersects  the scribbling. Any intersected
painted pixels leave quite a mess when dragging the canvas around upward.
Also, the scribbling on the right side of the canvas that is now outside of
the drawing bounds is visible whereas the stuff on the bottom is not. Should
this not be consistent?

Adrian