THE DEVELOPER EDITION / REVIEWED 26 OCT 2026

System behaviors on iPhone Duo: windows, media, motion, widgets, web

Original summary of Apple's written highlights of the iPhone Duo Group Labs (forum thread 847644, posted by an Apple DTS engineer in September 2026; sessions on demand), read 2026-10-07. API names and availability were checked against Apple's docs the same day. These are engineers' answers in a Q&A, not reference documentation: they can change, so test on a device before relying on one. Read this only when the question is about one of these topics. Back to the main reference.

Availability: CMBodyIdentifiable, CMMotionManager.deviceMotionBody, and WidgetFamily.systemExtraLargePortrait are iOS 27.0; UITabAccessory and AlarmKit are 26.0; the rest is older. Guard with #available.

1. Windows and multitasking

• iPhone Duo is the first iPhone that runs several instances of an app's UI, with iPad's scene model: each instance is its own UIScene. New windows can be created only on the inner display.
• UserDefaults and @AppStorage are app-wide, so every instance shares them. Keep per-window state in the scene.
• Two apps share the inner display in Split View. There is no Stage Manager and no free-form drag-to-resize: an app's size changes with the pose and with Split View only.
• There is no iPad-app compatibility mode (unlike Apple Vision Pro). How much of the display an iPhone app gets depends on the SDK it was built with (table in the main reference, §5).

2. State across folds

• Opening or closing is a size-class and trait change in the same scene; the process stays alive. Text being edited, the keyboard, and scroll position generally carry over, even when a LazyVGrid changes its column count, so there's usually no need to manage scrollPosition(id:anchor:) for this transition.
• For state that must survive a real scene disconnect and reconnect, save an NSUserActivity from stateRestorationActivity(for:).
• The hinge angle's update rate and precision are system policy and can change: don't build game mechanics that depend on either.

3. Media

• Picture in Picture (AVPlayerViewController or AVPictureInPictureController): open in landscape, people can pin the video to the top while the current app resizes vertically into the rest. Partially folded, the video grows to fill half the screen, so the app's height changes live. Picture in Picture moves to the outer display only when the device closes, and the system controls that move.
• With several instances, the active audio or video session belongs to the one that most recently started playback.

4. Core Motion

Each display faces a different way, so raw attitude doesn't match the UI. UIView conforms to CMBodyIdentifiable:

motionManager.deviceMotionBody = view          // nil = the device's hardware coordinate frame
motionManager.startDeviceMotionUpdates(using: .xArbitraryZVertical, to: .main) { motion, _ in
    guard let attitude = motion?.attitude else { return }   // already relative to `view` and the display it's on
}

• Core Motion transforms orientation, rotation rate, gravity, and user acceleration to match that view, and keeps doing so as it rotates or moves to the other display. Use one motion manager per view when two views need their own frame.
• Sources disagree about Split View: the Group Labs answer says that when the scene isn't full screen, body-relative orientation isn't available and data arrives in the device's default frame, while the CMBodyIdentifiable docs describe it working in Split View. Test both cases on a device before relying on either.
• You don't need to work out which face is up to lay out the UI: size classes and traits cover that.

5. Outer display, Live Activities, widgets

• An app can't draw on the outer display while the device is open. The ways in are system-controlled: a camera capture scene accessory (cameras), or AlarmKit, where the system presents the alert through AlarmKit, StandBy, or the Dynamic Island. For AlarmKit, make the alarm-management scenes resize for every size class and design the countdown Live Activity well.
• Live Activities: the outer camera's region grows into the Dynamic Island. Implement every presentation in ActivityConfiguration (compactLeading, compactTrailing, minimal, the expanded regions, the Lock Screen).
• Widget families: systemSmall, systemMedium, systemLarge, systemExtraLargePortrait, and the accessory families (accessoryCircular, accessoryRectangular, accessoryInline).
• Custom content beside the tab bar (a mini player, a status dot): attach a UITabAccessory through UITabBarController.bottomAccessory instead of drawing a custom tab bar, which never moves to the side. It lays out regular or inline, reported by UITabAccessory.Environment.

6. Web content

• Safari has the Viewport Segments and Device Posture APIs, but in iOS 27.1 both are experimental and off by default (Settings > Apps > Safari > Advanced > Feature Flags). Don't ship web content that depends on them.
• There is no device-detection API for iPhone Duo. Use responsive layout, as for any screen size.

7. Accessibility

• Dynamic Type behaves the same on both displays (preferredContentSizeCategory). On a fixed, one-screen layout, holding text at the largest standard size instead of letting it reflow or scale can fail the accessibility guidelines, and the short outer display makes this more likely: let content scroll.
• VoiceOver order follows standard containers and grouping. A custom layout that spans the fold may need explicit accessibility containers or sort priorities.
• Test Dynamic Type and contrast in each pose. Test VoiceOver directly on a device: Xcode 27.2 RC's Device Hub has an accessibility inspection limitation; see testing.