Feedback Assistant Issue FB8349717

Please provide a descriptive title for your feedback:

On iPadOS 14, UISplitViewController embedded in UITabBarController is always collapsed

Which area are you seeing an issue with?

UIKit

What type of feedback are you reporting?

Incorrect/Unexpected Behavior

Description

On iPadOS 14 (beta 4), UISplitViewController embedded in UITabBarController is always collapsed, preventing to display 2 or 3 columns side by side.

1. Create a UISplitViewController with a doubleColumn or tripleColumn style.
2. Set its preferredDisplayMode to .oneBesideSecondary (or .twoBesideSecondary for the tripleColumn style).
3. Embed the split view controller in a UITabBarController.
4. Launch the app on iPad.

• Expected behavior: both the primary and secondary view controllers should be displayed side by side (expanded interface).
• Actual behavior: only the secondary view controller is displayed in the UITabBarController (collapsed interface).

5. Print the split view controller horizontalSizeClass. It is .compact. It should be .regular. That is why the split view controller is collapsed instead of expanded.

This is a regression. Embedding a UISplitViewController in a UITabBarController works well on iOS 13 (and before).

Actual Behavior:

Expected Behavior: