Get an iOS app ready for iPhone Duo

iPhone Duo is Apple's first foldable iPhone: a 5.4-inch outer display and a 7.6-inch inner display sharing one aspect ratio, joined by a hinge the user can leave at any angle. An existing iPhone app runs on it unchanged — but it will look like an app that has not been thought about.

This skill is the entry point. Work the checklist, then hand off to the specialist skill for whatever the app actually does.

The one idea to hold onto

iPhone Duo is not a new idiom. It is a wider continuum of sizes.

Every mistake in this port comes from an app asserting something fixed: this idiom, this orientation, this screen, this width, this symmetric inset. Every fix is the same shape — replace the assertion with a question about the space actually available right now.

If you find yourself writing a branch that means "if iPhone Duo", stop. Write a branch on size class or available space instead, and it will be right on iPad, in Split View, under iPhone mirroring, and on whatever ships next.

Step 1 — Establish the baseline

Find out what the app is building against, because the SDK is the gate:

Built against	Behavior on iPhone Duo
Pre-iOS 27	Runs. Closed, it uses the space to the left of the status bar and camera. Open, it gets a familiar size and aspect ratio.
iOS 27 SDK	Extends left of the status bar area on the inner display.
iOS 27.1 SDK	Content reaches the screen edge, standard bars lay out vertically, and the reserved-region and arrangement APIs become available.

Check the deployment target and the SDK in the project, then say plainly which tier the app is in. Getting to the 27.1 tier is the single highest-leverage change; almost everything else in this skill assumes it.

Then open the app in the iPhone Duo simulator in Device Hub (Xcode 27.1) and drive the controls that open, close, rotate and fold it. Also try Split View by dragging the app to one side with the home indicator. Bugs here are visual and pose-dependent — you will not find them by reading code alone.

Xcode 27.1 also ships an App Resizability skill (renamed from the app modernization skill, now covering SwiftUI and iPhone Duo). Run it. It mechanizes much of Step 2.

Step 2 — Hunt the fixed assumptions

Grep the codebase for these. Each one is a real defect on this device:

Idiom and orientation branches. userInterfaceIdiom, isPad, any layout decision keyed off interfaceOrientation. The inner display does not honor supported interface orientations — it rotates regardless of what the app declared. Replace with size classes.

UIScreen.main. Ambiguous on a two-display device and slated for deprecation. Prefer no screen reference at all — use the environment, the trait collection, or the scene's bounds. For scale, traitCollection.displayScale. If you genuinely need the screen, reach it from the scene: window?.windowScene?.screen.

Symmetric inset math. This is the subtle one. On iPhone Duo the safe area and layout margins are routinely asymmetric — vertical controls sit on one side only, and which side depends on the pose and on Split View placement. Any arithmetic shaped like bounds.width - safeAreaInsets.left * 2 is wrong. Inset by the whole set instead and let each edge speak for itself.

Hardcoded widths and breakpoints. Fixed point values, device-width tables, "if width > 390" style thresholds. Target the two size classes: compact width on the outer display, regular width on the inner display.

Hand-rolled bars. A custom UIToolbar, UINavigationBar or UITabBar is invisible to the system's vertical-bar layout. Its content will not participate.

Step 3 — Adopt the standard containers

The cheapest path to a good port is letting system containers do the adapting. These are already fully pose-aware:

• NavigationSplitView / UISplitViewController — columns collapse to a single stack when closed, and appear tiled or as overlays when open.
• TabView / UITabBarController — adapt across every pose, laying out vertically where appropriate. On the inner display you can opt into a richer sidebar by setting the tab bar's preferred placement to .sidebar.
• NavigationStack, List, ScrollView — adapt to reserved regions on their own.
• Sheets, alerts, menus, popovers and context menus — reposition themselves around the fold automatically.

System components carry fold avoidance: they nudge interactive elements out of the curved region so buttons never land in the fold. You get that free by using them, and you owe yourself an implementation of it if you don't.

Step 4 — Get safe areas right

Four rules, in priority order:

1. Use container-provided bars for anything bar-shaped. They lay out outside the safe area and dodge the status bar, the Dynamic Island and the camera on their own. Horizontal bars contribute top/bottom insets; vertical bars contribute leading/trailing insets.
2. Keep interactive and legible foreground content inside the safe area. SwiftUI does this by default; in UIKit reference safeAreaInsets or constrain to the safe area layout guide.
3. Let backgrounds bleed past it — .ignoresSafeArea() in SwiftUI, view.bounds in UIKit — so artwork runs under bars and sidebars.
4. Handle every edge independently, and test in Split View on both sides.

For custom UI that needs to live outside the safe area without colliding with system UI, iOS 27.1 adds ReservedRegion (SwiftUI) and UIViewReservedRegion (UIKit). Reach for those rather than guessing at insets.

To match the screen's corner radius, use the iOS 26 concentricity APIs — ConcentricRectangle in SwiftUI, UICornerConfiguration in UIKit — which were updated for the shapes on this device.

Step 5 — Route to the specialist

If the app…	Use
has toolbars, tab bars, or overflow menus	iphone-duo-vertical-bars
has custom layout, split-like views, or content that lands in the fold	iphone-duo-adaptive-layout
wants hinge-driven effects, multiple windows, or dual-display UI	iphone-duo-hinge-and-scenes
captures photo or video	iphone-duo-camera
needs a design pass rather than a code pass	iphone-duo-design-review

Device numbers live in ../../reference/device-facts.md; every API name and its framework is in ../../reference/api-index.md.

Reporting back

Give the user the tier they're on, the concrete defects found with file:line, and what changes tier. Be honest about what needs a running simulator to confirm — pose-dependent layout bugs are not statically decidable, and claiming otherwise wastes their time.

Accuracy note

These APIs were announced in September 2026 Tech Talks while Xcode 27.1 was still rolling out. Treat the names in this repo as the shape of the API and confirm exact signatures against the SDK headers before promising a build works.