iPhone Duo app letterboxed or not full screen: why it happens and how to fix it

Updated 2026-09-12 · 5 min read · Markdown version

An iOS app is letterboxed on iPhone Duo because it was built with an SDK older than iOS 27; rebuilding with Xcode 27.1 and the iOS 27.1 SDK removes the bars, and adopting size classes, flexible layouts and the ReservedRegion API makes the app fill the inner display correctly.

You opened your app on an iPhone Duo and it sits in the middle of the inner display with dark bars around it, or it fills the screen but every control is stretched. Both are expected behavior for apps that predate the Duo SDK, and both are fixable in days, not months.

Why is my app letterboxed on iPhone Duo?

iOS decides how much of the inner display an app may use from the SDK the app was built against. Apps built with an SDK older than iOS 27 run in a compatibility window with limited screen usage, which appears as letterboxing. Apps built with the iOS 27 SDK extend past the status bar on the inner display. Apps built with the iOS 27.1 SDK reach the screen edge and get vertically laid out navigation and toolbar buttons. The letterbox is a promise from Apple that old apps still work; it is not a bug in your code.

Why is my app stretched instead of letterboxed?

If the app was already built with a recent SDK, it fills the display, but layouts written for a compact-width iPhone are now given regular width. A single column stretches across the 4.4-inch-wide inner display, buttons sized from the screen width become enormous, and lists show one item per row with empty space on both sides. Stretching is a size-class problem, not an SDK problem.

How do I fix it?

1. Rebuild with Xcode 27.1 and the iOS 27.1 SDK. This alone removes the letterbox and lets the app reach the screen edge.
2. Delete UIRequiresFullScreen from Info.plist if present. Apple deprecated the key in 2025 and the system ignores it once the app links against the iPadOS 26 SDK or later, so there is no longer a way to opt out of resizing or side-by-side multitasking. Leaving it in only misleads the next engineer.
3. Replace device idiom and orientation checks with size classes. The inner display is regular width and regular height; the outer display is compact width.
4. Give regular width a real layout: NavigationSplitView or UISplitViewController, adaptive grids, and a maximum content width for reading views.
5. Remove UIScreen.main.bounds and fixed frame sizes. Size from the container so the window can change live when the device opens or closes.
6. Adopt the ReservedRegion API for custom bars and edge-to-edge canvases so they do not collide with system UI, the fold or the camera.
7. Run every screen through all six poses in the Device Hub simulator and fix what you see.

How long does the fix take?

Effort estimates assume a team that already ships iOS regularly. After previous form-factor launches, reviewers noticed compatibility-mode apps within days, so it is reasonable to prioritize the rebuild first and the layout work in a follow-up release.