Four things break first, and none of them show up in a crash log. You only see them when someone folds the phone in the middle of a task.
1. Safe-area math that assumes both sides match
A lot of layout code finds the usable width by subtracting the left inset twice. On iPhone Duo that is wrong. Apple's Tech Talk warns that safe areas are often asymmetric, because vertical bars can sit on one side in landscape and in Split View multitasking.
The fix is to read each edge on its own, or take the width from the bounds after insets. It is a one-line change per screen, but you have to find every screen that does it2
2. Portrait lock used to skip landscape work
Many apps lock to portrait so the team never has to build a landscape layout. Apple states that the inner display does not honor your supported interface orientations. An app that has only ever drawn a portrait layout will have to draw something else on the inner display, planned or not.
Apple's advice is to stop checking orientation or device type for layout decisions and use size classes instead.
3. State lost during the fold
A size class is Apple's label for how much room your app has, compact or regular, in each direction. When the phone opens or closes, that label changes. If a form, a video, or a checkout flow resets when size classes change, the user loses their work.
Checkout is the costly one. Cart, payment, and in-app purchase flows should be the first flows you fold-test. People rarely rotate a phone mid-checkout. They will fold and unfold a Duo many times a day. Test your two or three most valuable flows by folding mid-step, not just by looking at static screens.
A fold is that same kind of lifecycle event. Every open and close changes the size class, and views that were never built to survive a change can rebuild and drop what they were holding. If your app has a remount bug today, iPhone Duo will find it several times a day.
4. Controls in the fold and camera surfaces
On the inner display, the fold is a reserved region. So is the area over the inner front camera when it is active.
Apple's containers route around both, but custom views need to ask for the reserved regions and move content out of them.
Camera apps have an extra problem. When someone closes or rotates the phone, your app may move to the other display, and the camera it was using may now point the other way.
If your app already plans for Live Activities sharing space with the Dynamic Island, you have a head start. On iPhone Duo, the vertical bar strip shares the edge with the status bar and camera in the same way.
All four of these are fixable without a rebuild. They also don't care whether your team built in Swift, Flutter, or React Native, and that is where cross-platform teams have their own questions.
