iPhone Duo folds. That means one app has to feel right on a short outer screen, a big inner screen, and every angle in between.

Here is what I learned doing this for ForkMesh.

The One Big Idea

You don't design a screen for each pose. You make one app that resizes well, and you let controls move to the side.

On Duo, tabs and toolbars leave the top and bottom and stand in a column under the camera.

The outer screen. The empty column by the camera now holds the controls, and the list gets the height back.

How the Side Column Works

A Live Activity can grow out of the camera and take space. When that happens, less important buttons move into the … menu.

Keep your main action and anything with a badge visible longest.

Use System Bars First

This was my biggest lesson.

I planned to draw this column by hand, then read that iOS moves standard toolbars and tab bars for you. Custom bars stay where they are.

When I put real toolbar items on a screen in ForkMesh, they moved into the column as real Liquid Glass.

If your app has a custom tab bar like mine, try this before you build anything.

The Big Screen Is Not a Stretched Phone

Held upright: The inner screen is the one pose that keeps normal top and bottom bars. There is enough height for them.

Respect the Fold

System sheets, alerts and menus move away from the fold by themselves. For custom views, use ReservedRegion.

Checklist

  • Build with the latest Xcode. Otherwise your app won't use the full screen.
  • Use size classes, not fixed widths. In my test, the outer screen reported compact width.
  • Measure the window, not the screen. A fold can resize your app at any moment.
  • Keep state when folding. I found code that closed the open page on fold. Typed text and open pages must survive.
  • Keep the same features in every pose. Only the layout should change.
  • Give every toolbar item an icon and a title. Items with only text or a custom view won't move to the column.
  • Test short screens. The outer screen clipped my onboarding headline behind a button.
  • Check Split View. On the left half, controls move to the left edge.

Prompt You Can Use

Paste this into your AI coding tool, inside your project.

iPhone Duo Prompt 

Make this iOS app work well on iPhone Duo. Work in small steps and show me each result. First, read the code and tell me: - how the app handles navigation, tab bars and toolbars - which bars are custom and which are system - any fixed widths, screen-size checks or device checks - any code that resets state when the window resizes Then follow these rules: 1. Build with the latest Xcode and test on the iPhone Duo simulator. 2. Prefer system containers: TabView, NavigationStack, NavigationSplitView and .toolbar. Test one screen first, because custom bars do not move to the side. 3. If a custom bar must stay, use toolbarVerticalEdge to find the side and ReservedRegion to avoid the camera and fold. 4. Use size classes and safe areas. Remove fixed sizes and do not use the device model for layout. 5. Outer screen: one pane, controls in the side column. Back at the top, then page actions, then overflow. Tabs stay at the bottom. 6. Inner sideways: show list and detail together, with controls on the same side as the outer screen. 7. Inner upright: keep normal horizontal bars. 8. Half open: keep buttons out of the fold and let it divide the panes. Scrolling content may cross it. 9. Keep state through every fold and resize: open page, scroll position, typed text and running tasks. 10. Give each toolbar item an icon and a title. Set visibility priority so the main action and badged items stay visible longest. 11. Keep the same features in every pose. Keep other iPhones unchanged. 12. Check every screen for clipped text and for controls hidden by the side column, including onboarding, sheets and Split View. At the end, list what you verified, what you could not test, and anything I should check by hand on Duo.