on-paint in canvas%
Called when the canvas is exposed or resized so that the image in the canvas can be repainted.
When
on-paint is called in response to a system expose event and only a portion of
the canvas is newly exposed, any drawing operations performed by
on-paint are clipped to the newly-exposed region; however, the clipping region
as reported by
get-clipping-region does not change.
( -> voidsend a-canvas on-paint)
Calls the procedure supplied as the paint-callback argument when
the canvas% was created.