Apple’s first foldable iPhone, iPhone Duo, may show substantial black borders around apps that have not been updated for its displays. The company has outlined three compatibility levels tied to the SDK used by an app, with the oldest apps potentially confined to a narrow vertical area on the internal screen rather than filling the available panel.
Apps built with SDKs predating iOS 27 are the most limited. Apple’s demonstration shows that these apps can occupy only a central portion of the inner display, leaving the surrounding space black. The outer screen presents a less severe result, but such apps still do not make full use of its aspect ratio. Apple is avoiding forced stretching, which could distort interface elements or disrupt functionality.
Apps built with the iOS 27 SDK can be resized and reorganized to use far more of the screen. Apple’s examples indicate that these apps can present multiple columns on the inner display. Full use of the available area, however, is reserved for apps optimized with the iOS 27.1 SDK, which can adjust navigation and layout according to the active screen and orientation.
Developers will need to do more than enable resizing. Their interfaces must handle transitions between the external display and the unfolded screen, including when the handset is opened while an app is already running. Apple will continue to run iPhone versions of apps on iPhone Duo rather than allowing iPad apps, despite the larger workspace offered by the internal display.
The arrangement echoes the early iPhone X period in 2017, when many apps temporarily retained black bars while developers adapted to the phone’s new display shape and screen cutout. On iPhone Duo, how quickly the experience reaches the full-screen iOS 27.1 level will depend on developer updates.
