This post is from the Activity iPhone Duo SwiftUI Q&A.
As a workaround we used xcrun devicectl device appResize. It works: setting 951×669 makes our layout classifier report wideSplit and 669×951 reports tallSplit, confirmed from the app's own debug readouts. Two behaviours we had to discover empirically: the resize must be applied while xcodebuild test is already running, because xcodebuild's own app launch resets a size set beforehand; and appResize start needs roughly six seconds to establish the session, failing at two seconds with "not currently hosting a resizable app session". Is appResize the intended mechanism for exercising foldable scene sizes in tests, or is there a first-class API we should be using instead?