Explainer
Why some apps have black bars on iPhone Duo
The three ways iPhone Duo displays an app on its inner screen, why it depends on the SDK, and what you can do about it.
Updated
Open iPhone Duo and its 7.6-inch inner display is closer to a small tablet than a phone. Every iPhone app still runs — but not every app knows what to do with that much room. Until a developer updates their app, iPhone Duo keeps it in a phone-sized window, and the rest of the screen stays black.
It depends on which SDK the app was built with
Every app is compiled against a specific version of Apple’s software development kit (SDK). iPhone Duo checks that version and picks one of three display modes:
| Built with | What you see on the inner display |
|---|---|
| iOS 26 SDK or earlier | Black bars. The app runs inside a phone-sized safe area on a black background. |
| iOS 27 SDK | Partly adapted. The app extends across much more of the display, but some space is left unused. |
| iOS 27.1 SDK and later | Full screen. The app reaches the edges, and standard toolbars can move to the side of the screen. |
The iOS 27.1 SDK arrived with the first Xcode 27.1 beta on September 18, 2026 — about five weeks before iPhone Duo ships. That’s why many apps will still show black bars on launch day: developers can’t submit a fully adapted build until they’ve had time to test it, and adapting a layout properly usually takes more than a recompile.
The outer display is unaffected
The 5.4-inch outer display behaves like any other iPhone screen. If an app has black bars inside, you can always close iPhone Duo and use it on the outside, where it looks exactly as it would on a regular iPhone.
Black bars aren’t a bug
A letterboxed app is fully functional. Buttons, text and gestures work as normal; you’re simply not getting the larger canvas. It’s the same compromise Apple has used before — iPhone apps on iPad run in a similar compatibility window.
What you can do about it
- Keep your apps updated. The bars disappear as soon as the developer ships a version built with the newer SDK. No setting on your side changes this.
- Use Split View. A letterboxed app wastes less space when it shares the inner display with a second app. How Split View works →
- Tell the developer. A short, polite review or support message that mentions iPhone Duo does get noticed.
- Tell us. If you own iPhone Duo, report how an app looks so other people know before they buy.
Which apps still have black bars?
We track the most popular apps and update each page when there’s evidence. See the full list of apps with black bars, or search for any app at the top of this page.