I still try to optimise my Gamelan application and stumbled over this:
I've a canvas based custom control with a picture as backdrop. To highlight parts of the picture (played keys of the instrument) I draw semi-transparent rectangles over it. Now to un-highlight, running on mojave I can just do a ClearRect to remove the drawn rectangle, but on high sierra and before, this clears out the backdrop too.
Is this a known/documented behaviour, or a bug either in mojave or high sierra?
I can instead of ClearRect just draw another picture with 100% transparency, but ClearRect would be faster.