Have been struggling to resolve the same problem for about two days. Turned out the application should not require full screen when supporting multiwindow/multiscenes:
TheDreamsWind
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After digging a little deeper into The Objective-C Programming Language documentation, I actually found the exact answer to such a scenario:
Protocols can’t be used to type class objects. Only instances can be statically typed to a protocol, just as only instances can be statically typed to a class. (However, at runtime, both classes and instances respond to a conformsToProtocol: message.)
Which means that it's just not supported and I should implement this differently