# iPhone Duo Adaptive Layout: Design Across Displays

> Keep the user’s task recognizable while the available window changes. Plan what stays visible and what rearranges, then use current scene geometry, size classes, safe areas and reserved regions in implementation. A wider iPhone Duo window is not a separate iPad target or a fixed screen template.

Author: Plurigent
Content version: 0.13.1
Source review: Oct 4, 2026
Applies to: iOS 27.1; SwiftUI; UIKit; WKWebView; outer and inner displays; Split View
Verification status: Selected Apple video text and the Apple Staff iPhone/iPad-target reply reviewed October 4, 2026. API names remain references, not an availability matrix or proof of compilation. No native layout or device measurements were performed.
Original page: https://plurigent.com/tutorials/iphone-duo/adaptive-layout

Plan iPhone Duo compositions around the current window, navigation and task. Distinguish display examples from runtime size classes, reserved regions and native adaptation.

Scope: official sources and practical guidance. Runtime measurements and hands-on observations are labeled separately and should be rechecked against the stated versions.

Reading scope: For Designers / For Developers
Content formats: Guide

## How should layouts adapt on iPhone Duo?

Evidence type: Official information

An app can move between displays, orientations and window sizes without starting a new user task. Treat full-display layouts as examples, not the only possible app bounds. The current scene and system containers determine the usable space.

A full-window inner display can offer a regular-width layout, but an app in a smaller window must respond to its actual traits and dimensions. Do not infer every window’s size classes from the hardware name.

Apple Staff confirms that iPhone Duo uses the .phone idiom. Supporting Duo does not require adding an iPad target; supporting a regular-width iPhone window and choosing to support iPad are separate decisions.

[A03 · Prepare your app for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111461/)
[Q02 · Apple Staff · iPhone Duo and iPad target support](https://developer.apple.com/forums/thread/847945)

## Use the current scene, not global screen assumptions

Evidence type: Official information

Apple advises against using the main screen as a layout source on a device with multiple displays. Prefer SwiftUI environment values, UIKit trait collections, the current scene bounds, or a screen obtained from the relevant window scene.

The inner display does not follow supported interface orientations in the same way as the outer display, so layout branches based on interface orientation are less robust than size-class and available-space decisions.

[A03 · Prepare your app for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111461/)
[A05 · Leverage multiple displays and scenes on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111464/)

## Let system navigation and presentation containers adapt first

Evidence type: Official information

NavigationSplitView, UISplitViewController, TabView, and UITabBarController already adapt across iPhone Duo poses. Columns can collapse when closed and appear tiled or overlaid when open. Sheets, popovers, context menus, and alerts also adapt.

Before building custom pose logic, verify whether a standard container already expresses the desired behavior. Custom bars should use safe areas and reserved regions rather than hard-coded offsets.

[A03 · Prepare your app for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111461/)
[A02 · Design for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111466/)

## When do controls move into the vertical bar?

Evidence type: Official information

Standard navigation containers contribute their toolbar and tab items to the vertical bar. A standalone custom bar does not gain this behavior automatically. Items need an icon and a useful title so the system can present them in different locations.

Display-edge placement matters: in a split view, the detail column can participate while other columns stay horizontal. Inspectors do not receive a second vertical bar.

[A17 · Raise the bar with iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111462/)

## Choose the layout API for the job

Evidence type: Official information

Keep the scene, bar and fold decisions separate. Use an arrangement only when the relationship between primary and secondary content benefits from it.

| Need | SwiftUI | UIKit |
| --- | --- | --- |
| Primary/secondary layout | ArrangementView | UIArrangementViewController |
| Reserved-region geometry | GeometryProxy.reservedRegions(...) | UIView.reservedRegions(...) |
| Custom toolbar item axis | axisBehavior(...) | UIBarButtonItem.axisBehavior |
| Overflow importance | visibilityPriority(...) | UIBarButtonItem.visibilityPriority |

[A18 · Preparing your app for iPhone Duo](https://developer.apple.com/documentation/technologyoverviews/preparing-your-app-for-iphone-duo)
[A17 · Raise the bar with iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111462/)

## Design around the fold with reserved regions and displacement

Evidence type: Official information

When partially folded, the hinge can become an active division reserved region. Apple’s displacement pattern moves the existing element or container into an appropriate usable region while preserving its relationship to surrounding content.

For more structured two-region layouts, Apple’s arrangement APIs can express split and overlay presentations. Do not move every element independently simply because a fold exists; preserve meaningful groups and avoid unnecessary motion.

Use the reported layout arrangement and reserved regions for layout decisions. A hinge-angle value can drive a visual effect, but an invented angle threshold is not a substitute for the system’s usable-region information.

[A04 · Strike a pose with adaptive layouts on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111463/)
[A05 · Leverage multiple displays and scenes on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111464/)

## Preserve the task while the layout changes

Evidence type: Practical guidance

Treat layout state and business state as different layers. Opening the device may change columns, bars, safe areas, and viewport dimensions without changing the selected item, draft text, pending request, media position, or current workflow step.

For WKWebView, keep the page instance alive when possible, update the native container, and let the web layout reflow inside the supplied area. A resize callback should not be an implicit signal to reload data or recreate the page.

For a design review, try one list-to-detail task with the same realistic content in a compact and a wider composition. Mark the selected item, unfinished input and scroll position that must survive. This is a Plurigent exercise, not a tested platform result.

- Keep selection and navigation context stable when columns appear or collapse.
- Keep form values and validation state through size changes.
- Reposition sheets and overlays without duplicating requests.
- Recompute safe areas and viewport values instead of caching device-model constants.
- Test keyboard-visible transitions, not only empty static screens.


## Common adaptive-layout anti-patterns

Evidence type: Practical guidance

Avoid device-name checks, fixed screen-width tables, mirrored safe-area assumptions, global UIScreen.main layout logic, and component recreation tied directly to width thresholds. These approaches make one screenshot look correct while weakening the next configuration.

Use explicit layout rules and observable test cases instead: which content has priority, what changes at compact versus regular space, what remains the same, and what state must survive.


## Sources & review scope

- [A03 · Prepare your app for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111461/) — Oct 4, 2026; Selected preparation transcript passages reviewed October 4 for system containers and task continuity. No native SDK compilation or device/runtime measurement.
- [Q02 · Apple Staff · iPhone Duo and iPad target support](https://developer.apple.com/forums/thread/847945) — Oct 4, 2026; Written answer reviewed for .phone idiom and optional iPad target. Not a compiled target configuration or device-trait measurement.
- [A05 · Leverage multiple displays and scenes on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111464/) — Oct 4, 2026; Selected multi-display transcript passages reviewed October 4 for layout versus hinge-driven effects. No multi-scene, camera or hinge-sensor execution test.
- [A02 · Design for iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111466/) — Oct 4, 2026; Official chapter index and selected design transcript passages reviewed October 4. Seven existing chapter offsets match. No YouTube playback, subtitles or native-device verification.
- [A17 · Raise the bar with iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111462/) — Oct 4, 2026; Official six-chapter index and selected toolbar transcript reviewed October 4. No playback, subtitle, compiled sample or native behavior check.
- [A18 · Preparing your app for iPhone Duo](https://developer.apple.com/documentation/technologyoverviews/preparing-your-app-for-iphone-duo) — Sep 23, 2026; Reviewed the official Markdown documentation on resizing, scene geometry, bars, arrangements, reserved regions and camera handling. No native implementation tested.
- [A04 · Strike a pose with adaptive layouts on iPhone Duo](https://developer.apple.com/videos/play/tech-talks/111463/) — Oct 4, 2026; Selected adaptive-layout transcript passages reviewed October 4. Layout arrangements/reserved regions are references, not a new API-availability or native execution proof.

## Keep reading

- [iPhone Duo Safe Area Insets & Layout Margins](https://plurigent.com/tutorials/iphone-duo/safe-area)
- [iPhone Duo WKWebView Guide: Viewports, Safe Areas & Keyboards](https://plurigent.com/tutorials/iphone-duo/webview)
- [iPhone Duo App Testing Checklist](https://plurigent.com/tutorials/iphone-duo/app-testing)