UITabBarController bottom accessory doesn't resize properly when horizontal size class changes from compact to regular

A bottom accessory view is set on the UITabBarController. When changing the window size either by dragging the resizing grip or when going to portrait mode, the accessory view shrinks down to the smaller width. When resizing the window to make it larger, the accessory view doesn’t resize to the full available width.

During a workshop setup by Apple, folks from Apple told me that the view set as the content view of the UITabAccessory should not have its frame changed, either by using Auto Layout or by setting the frame. It seems logical since the view in the bottom accessory is supposed to resize accordingly to several factors, like when going inline inside the tab bar.

Am I missing something? Maybe there is additional setup required not mentioned in the dedicated video.

Feedback is FB19017330. It contains a sample project and videos demonstrating the issue. Reproduced on Xcode 26 beta 6 (17A5305f)

I copy and paste the sample code that setups the bottom accessory for good measure.

final class MiniPlayer: UIView {

    let label = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)

        label.text = "Mini Player"
        label.numberOfLines = 0
        label.textAlignment = .center
        addSubview(label)

        label.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: centerXAnchor),
            label.centerYAnchor.constraint(equalTo: centerYAnchor)
        ])
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

// MARK: - View Controller

class ViewController: UIViewController {

    let rootTabBarController = UITabBarController()
    let miniPlayer = MiniPlayer()

    override func viewDidLoad() {
        super.viewDidLoad()

        let items: [UITab] = [
            UITab(title: "Tab 1", image: UIImage(systemName: "archivebox.fill"), identifier: "tab-1") { _ in
                UIViewController()
            },
            UITab(title: "Tab 2", image: UIImage(systemName: "books.vertical.fill"), identifier: "tab-2") { _ in
                UIViewController()
            },
            UISearchTab { _ in
                UIViewController()
            }
        ]
        rootTabBarController.tabs = items

        rootTabBarController.view.backgroundColor = .secondarySystemBackground

        view.addSubview(rootTabBarController.view)
        rootTabBarController.view.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            rootTabBarController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            rootTabBarController.view.topAnchor.constraint(equalTo: view.topAnchor),
            rootTabBarController.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            rootTabBarController.view.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])


        rootTabBarController.bottomAccessory = UITabAccessory(contentView: miniPlayer)
    }
}