Guide · Updated
Why some apps show black bars on iPhone Duo
Why apps open in a small window with black bars on the iPhone Duo inner display, which apps are affected, and what has to happen before they fill the screen.
What you are seeing
On the inner display, an app that hasn't been updated for iPhone Duo opens in a phone-sized window in the middle of the screen, with empty space on both sides. This is called letterboxing.
Folded, on the outer display, the same app looks normal, because the outer screen is a regular iPhone size.
Why it happens
An app can only use the full inner display if the developer builds it with the iOS 27.1 SDK or later. Apple started accepting those builds on October 5, 2026, so every app whose latest version is older than that is letterboxed.
Letterboxing keeps old apps usable instead of stretching layouts that were never designed for a wider screen.
When will it be fixed?
Only the developer can fix it, by shipping an update built for Duo. There is no date for that: big apps often update within weeks, smaller ones can take months.
You can follow every update on our update tracker, and tap "I want this" on an app page to show developers that Duo owners are waiting.
Black bars vs. stretched vs. built for Duo
These are the three things you will see on the inner display:
- Letterboxed: a phone-sized window with empty space around it.
- Stretched: the app fills the screen, but it is the iPhone layout made wider.
- Built for Duo: the app uses the extra width, for example with a sidebar or two panes.
Can I force an app to fill the iPhone Duo screen?
We haven't found a setting that does this. The change has to come from the developer in an app update.
Does a letterboxed app work normally otherwise?
Usually yes. Letterboxing changes the size of the window, not what the app can do. If an app crashes or freezes, that is a different problem, and owners report it as "Broken".