I have been able to reproduce a problem with running a SwiftUI iOS app project converted in Xcode 13 beta for use in Xcode 12. Doing so is consistently resulting in the main view's top and bottom being cut off and centered within a black background (I believe this is referred to as "letterboxing").
Here are the steps to follow:
- Use xcode-select to set your environment for the Xcode 13 beta and launch it.
- Create a new SwiftUI app
- In its ContentView file, add some content
- Run the project in the iOS Simulator
- Select the project in the Project navigator and in the File Inspector, change the Project Format dropdown menu to Xcode 11.4-compatible or Xcode 12-compatible.
- Quit the Xcode beta and the Simulator.
- Use xcode-select to set your environment for Xcode 12.5 and launch it.
- Open the project you created in Xcode 13 beta and run it in the iOS Simulator.
Your simulator screen should look something like the screenshot below.
I have not been able to find any way of fixing this problem once the file has been opened in Xcode 12. After relaunching Xcode 13 and rerunning it there the black borders do not appear (but there are other problems I've run into that make me want to continue development in Xcode 12 for the project where I ran into this problem.)