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The iPhone Duo arrives today, and some Flutter apps will open on it boxed in by black bars

The foldable iPhone runs older apps in a small window. What to change in a Flutter app, what the framework provides, what it lacks, and what to watch for.

ProCat Solutions iPhone DuoFlutteriOS 27.1FoldableMobile development
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The iPhone Duo arrives today, and some Flutter apps will open on it boxed in by black bars

Open the new iPhone Duo today, launch an app that was last updated a year ago, and you get a smaller window with black bars in the middle of the 7.6-inch inner display. Apple does this on purpose. On the foldable, only apps built with the iOS 27.1 SDK fill the whole inner screen. Everything else runs as if it were on a smaller iPhone.

Apple unveiled the iPhone Duo on September 9. Pre-orders opened on October 16, and according to MacRumors, delivery estimates slipped to early 2027 within minutes. The device measures 5.4 inches closed and 7.6 inches open, and prices start at $1,999. From April 2027, every app submitted to the App Store must include iPhone Duo screenshots.

A half-folded foldable phone on a desk, showing a two-column app

What native apps get for free

Apple says apps built from standard SwiftUI and UIKit components adapt on their own. Controls move to the side, within reach of the hand. On the inner display, layouts split into several columns. In the half-folded position, buttons shift away from the crease. The iOS 27.1 SDK adds new APIs for splitting content into two halves and for handling reserved areas, and a hinge API reports the angle of the display. In the new Device Hub in Xcode 27.1, the simulated Duo can be opened, closed and folded halfway.

Flutter’s disadvantage

Flutter does not use the system’s controls. It draws the interface itself, so it gets none of this automatic adaptation. The framework has a designated place for fold data, the MediaQuery.displayFeatures field. According to the dart:ui documentation, that field is only populated on Android. On iOS it stays empty.

A proposal to close the gap is already on Flutter’s GitHub page (#192515), and community packages have appeared, such as duo_display and duo_flex. They read the hinge position, the fold state (closed, half-open, fully open) and the camera cutout from native UIKit data, and put them where Flutter code already looks for them. The packages are new. In a production app, this layer is best treated as your own code, something you understand and can fix if the next iOS update changes something.

A developer testing on an unfolded foldable phone, with a laptop showing code beside it

For and against

The case for Flutter:

  • One codebase produces the Android foldable, the standard iPhone and the iPhone Duo versions. For Android foldables, displayFeatures already works today.
  • LayoutBuilder and responsive layouts work the same way on the inner display. The two-column logic is written once.
  • The interface is pixel-identical on every platform, which matters for brand-sensitive apps.

The case against:

  • The automatic adaptation native apps get has to be written by hand: moving controls, avoiding the crease, splitting into columns.
  • Hinge and fold data currently require a community package or your own native bridge.
  • New iOS features usually reach Flutter late. If you want to be first with a Duo-specific feature, native is faster.

What to watch for

  • Rebuild with the iOS 27.1 SDK. Without it, the app runs in a small window on the inner display. This is the cheapest and most important step.
  • Don’t decide on width alone. On the iPhone Duo, width by itself does not tell you the size class. It is worth passing through the native size class as well.
  • The crease. In the half-folded position, keep buttons, text fields and important text off the fold.
  • Continuity. When the device is opened or closed, the app should not restart a screen or lose typed input or scroll position.
  • Test in every posture. Closed, open, half-folded and rotated, in the Device Hub and, if possible, on a real device.
  • Screenshots by April. App Store submissions require Duo screenshots, so plan them into the next release.
Two hands unfolding a thin foldable phone

Where we can help

We build mobile apps in Flutter and natively. Moving an existing Flutter app to the Duo takes three steps with us: rebuilding with the iOS 27.1 SDK and removing the letterbox, a small native bridge for hinge and fold data, and finally the two-column layout and crease handling on the key screens. If the app runs on a SaaS backend, we handle that too: mobile and SaaS development.

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