Sure, they said you can just click the checkbox. But there have been huge rough edges, like nothing about collection views working without crashing, until beta 11. Document-based apps get a blank window behind the open dialog box, and the documentation for how to properly handle multi-screen displays or multiple windows is sparse. However, coding for UIKit is soooooo much easier than AppKit, that I'm able to re-build better interfaces with macCatalyst in a fraction of the time it took me to build my AppKit based app. Except pixel-aligned graphics, since the scaling factor from pixels on iOS to macOS is the ratio of two prime numbers.