This post is from the Activity iPhone Duo UIKit Q&A.
In our app we have situations where the user may be blocked for security reasons and we currently present a full screen modal - can we fill the screen with a modal in such a situation on Duo if the app is full screen on the inner display?
You can fill the screen with a modal by setting modalPresentationStyle = .fullScreen in UIKit and using .fullScreenCover in SwiftUI.
If your app is placed side-by-side in Split View with other apps, the modal will only fill your app’s active window scene and will not cover the entire physical display.