P: Undoing the first paint action in a Mask selects the Art Layer.

Stunning thank you! It didn't work initially, but after dumping the preferences it is working a treat. Perfect! 

I'm working with CC version 20.0.1 (the most recent version) & for the last couple of months I've been experiencing an annoying problem where if I hit Command-Z after painting on a mask, it will cause my curser to jump to the corresponding pixel layer.  Therefore once I start painting again, I am painting a color onto the pixel layer instead of the mask.  Also, it sometimes causes my foreground/background paint (black or white) to switch, so when I go back to the layer mask to start painting it will paint the opposite of what I intended.

Thanks Michael & Earth! I'll have engineering investigate this. Workaround, as you noted in the other thread, is to enable legacy undo under Edit > Keyboard Shortcuts....

Jeffrey see my post https://forums.adobe.com/message/10730632 the behaviour is back i have included a video.  Only way to fix is by changing back to legacy undo

Hey Jeff, i made a post on the other forums called 'Multi-Undo bad behavior is back,' which has a few screencaps illustrating the issue.

Earth, Dylan, I'm not able to reproduce this. Could you provide steps or video of your steps?

Ohh wow, thanks, that helps!

But, I must admit, the new undo feature was the only frustrating change Adobe made that I really supported. I'm all for frustration when it's about synchronising behaviour across their software suite, but when it comes to changing behaviour for no reason, like what they did with transform tools, that kills me!

Yes, they had fixed this bad behavior in a previous release and then the new undo has brought it back. There's a pref in keyboard shortcuts to change back to legacy undo... see if that helps.