I've been testing the Scratch/Snap interface, working on the Mac, with RoboPaint running.
Things seem to mostly be working when RoboPaint is the "foreground" app-- not running in the background.

When a different application is in the foreground-- say, Chrome running Snap, or even just a text editor, painting continues, but with a delay of about 0.5 seconds between any two movements (line segment, lift/lower, etc) of the machine. For example, swishing the pen in the paint takes about 10 seconds.

In practice, this means that the Scratch/Snap interface only barely runs at all-- it feels malfunctional. If one switches back to RoboPaint in the foreground once the job is running, then everything works fine, but I don't think that this is a good solution.