From Size Class to Available Space: Is horizontalSizeClass Still Reliable?

Starting with WWDC 26, when an iPhone app is mirrored to a Mac through iPhone Mirroring, its window can be freely resized. At the same time, iPhone-only apps running on iPad will also enter a resizable environment. Even without updating a physical device to a beta system, developers can already experience this change through Xcode 27 previews or the iOS 27 simulation environment in Device Hub.