This post is from the Activity iPhone Duo UIKit Q&A.
On the unfolded inner display our root view controller’s horizontalSizeClass becomes .regular, the same as iPad. Folded, it is .compact. Is that the supported way to tell the two displays apart, or is there a Duo-specific trait, screen idiom, or UIWindowScene property we should use instead of size class?
Size classes are the right path to take here, don't think in terms of the specific screen, just the available space you have to do your layout. For example, consider the multi-tasking support where two apps can be open on the inner display, and ensure you're eating that your layout adaptations works well in that configuration as well.
