Why some apps have black bars on iPhone Duo

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:

The iOS 27.1 SDK arrived with the first Xcode 27.1 beta on September 18, 2026 — about five weeks before iPhone Duo went on sale. That’s why most third-party apps still had black bars when reviewers tested iPhone Duo the week before launch, even as apps like Slack and Instagram were updated during that week.

On the outer display, it’s a normal phone app

The 5.4-inch outer display works like a regular iPhone screen. An app that isn’t optimized runs at normal phone size there, with a thin black strip on the right where optimized apps put their controls.

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

There’s no setting that stretches an app to fill the inner display. These are the options reviewers found:

• Tap the zoom button. Apps that aren’t optimized show an iPad-style zoom button that enlarges the app to fill more of the inner display. Text and buttons look oversized, but you see more of the app.
• Rotate the app. If the app supports landscape, you can switch between its portrait and landscape layouts.
• Close the phone. On the outer display the app looks as it would on any iPhone.
• Use Split View. A letterboxed app wastes much less space when it shares the inner display with a second app. How Split View works
• Keep your apps updated. The bars disappear for good as soon as the developer ships a version built with the iOS 27.1 SDK.
• 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.

How to find apps that fill the screen

On the App Store, optimized apps carry a Made for iPhone Duo badge on their product pages, and Apple opened collections of iPhone Duo apps and games on launch day. On WorksOnDuo, see the apps that are built for Duo or run full screen.

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.