I'd love to know too: I discovered it "emprically", as they say; i.e. much hair-pulling, swearing and cursing was involved. Lock/unlockfocus was the recommended way of doing things and our legacy app was littered with them. Oh, it's building with the newer SDK that triggers the isuue - had you built your app with, say, Xcode 9, it would have been OK on 10.14+ (you might have other issues, of course).
