When iPhone Duo is unfolded completely flat to 180°, ReservedRegion(.division) becomes inactive or reports empty geometries. At this point, both the flat inner display and the closed cover display present continuous rectangular surfaces.
We currently infer the inner display by evaluating the asymmetric system safe area signature (specifically the presence of the 82 pt top status band versus the trailing lateral cluster on the outer display). Is relying on asymmetric safe area insets the intended pattern to discriminate the active display surface in SwiftUI, or is there a preferred platform API to identify whether a window is hosted on the internal foldable panel versus the cover glass?
Attempting to deterministically detect the inner or outer display is the wrong approach. There is intentionally no API to determine this.
I'd suggest changing your perspective to think about how to best lay out your app based on the space it has available. Specifically look at size classes and things like the available space for the immediate view you are considering during layout.
"Prepare your app for iPhone Duo" has some great tips of how to mentally approach this in a way that will allow your iPhone app to resize in iPhone Mirroring, or when running on an iPad:
https://developer.apple.com/videos/play/tech-talks/111461/
"Strike a pose with adaptive layouts on iPhone Duo" also has great conceptual content of how to consider designing and laying out your app:
https://developer.apple.com/videos/play/tech-talks/111463/
Thinking about how to generally layout your app with any available space, means you can adapt from iPhone, to iPhone Duo, to iPad, and other devices.
At that point, the code you need to write for iPhone Duo is small. Perhaps adopting an ArrangementView that responds to the hinge, or if you have something more manual needed, using Reserved Regions.
