在进行后滑动过渡时，半透明导航栏已破坏动画

Question

我正在尝试实现透明导航栏，在滚动表视图时逐渐变为纯色。为此，我将导航栏设置为透明并在其后面放置一个uiview（此UIView也将覆盖状态栏）。然后我可以在此视图上为alpha设置动画。但是，如果我按下一个新的视图控制器，然后尝试向后滑动过渡动画似乎很奇怪。右上角有一个黑色方块，如此（点击图片查看动画！）：

代码是这样的：

override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
        navigationController?.navigationBar.shadowImage = UIImage()
        navigationController?.navigationBar.isTranslucent = true
}


override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(true)
        if !navCoverViewSet {
            guard let navBarHeight = navBarHeight() else { return }
            navCoverView.backgroundColor = UIColor.blue.withAlphaComponent(0.4)
            navCoverView.translatesAutoresizingMaskIntoConstraints = false
            navCoverView.heightAnchor.constraint(equalToConstant: navBarHeight).isActive = true
            navCoverView.widthAnchor.constraint(equalToConstant: view.frame.size.width).isActive =  true
            navCoverView.bottomAnchor.constraint(equalTo: view.topAnchor, constant: 0)
            view.addSubview(navCoverView)
            navCoverViewSet = true
        }
    }

    func navBarHeight() -> CGFloat? {
        guard let navBarHeight: CGFloat = navigationController?.navigationBar.frame.size.height else { return nil }
        let statusBarSize: CGSize = UIApplication.shared.statusBarFrame.size
        let statusBarHeight: CGFloat = min(statusBarSize.width, statusBarSize.height)
        return navBarHeight + statusBarHeight
}

为什么会发生这种情况的任何想法？我会喜欢这方面的建议。谢谢！