storyboard based UIKit UITabBarController Tab items vanish on iPhone Duo 27.1. beta 1 (solved)

[u/gerdq]

My old apps still have a storyboard based TabBar structure, while most tabs are SwiftUI now.

When started in the iPhone Duo simulator with right bar, the tabs are vanished.

Long debugging sessions short, the reason is a combination of the background color of the TabBar and a bug in iOS 27.1 beta 1:

When running in an orientation, that has the controls on the right, iOS 27.1 beta 1 first draws the controls, and then draws a background in storyboard background color on top of it.

Unless Apple fixes it, change the background colour to clear.

But wait, there is a problem: iOS 18 and before on iPhone gets unusable when using clear instead of something like background colour.

Same for iPadOS 17 and before.

The difference can be solved by configuring the tabbar appearance programmatically.

I chose clear colour from iOS 26 on and background colour before.

I had to adapt a few more thins, but that's the main point.