Re: NSManagedObjectContext's undoManager is nil on macOS

This behavior changed in 10.12 for applications linked against that SDK or later. It’s listed in the “Behavioral changes” section of the Core Data release notes for that year:

Re: is there a way to determine if an NSButton is toggleable ?

NSButton is interesting in that it separates out the state (e.g. `isHighlighted` and `state`) from the presentation of that state. The state manipulation behavior is totally uniform, i.e. the `state` property always toggles between Off and On when any button is clicked. Most buttons aren't