SplitOrFlipViewController_willTransition.swift

override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
    super.willTransition(to: newCollection, with: coordinator)
    
    coordinator.animate(alongsideTransition: nil) { _ in
        switch displayMode {
            case .sideBySide:
                navigationItem.rightBarButtonItem = nil
                break
            case .one:
                navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Flip", style: .plain, target: self, action: #selector(flipAction))
                break
        }
    }
}