UIButton masksToBounds issue

gistfile1.swift

class TestButton: UIButton {
    init() {
        super.init(frame: CGRect.zero)

        self.layer.masksToBounds = true
        self.layer.borderColor = UIColor.black.cgColor
        self.layer.borderWidth = 1 / UIScreen.main.scale

        self.layer.cornerCurve = .continuous

        self.setTitle("Test", for: .normal)
        self.setTitleColor(.black, for: .normal)
    }

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

    override func layoutSubviews() {
        super.layoutSubviews()
        self.layer.cornerRadius = self.bounds.height / 2.0
    }
}

class TestViewController: UIViewController {
    init() {
        super.init(nibName: nil, bundle: nil)
    }

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

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.backgroundColor = .white

        let testView = TestButton()
        testView.translatesAutoresizingMaskIntoConstraints = false

        self.view.addSubview(testView)
        NSLayoutConstraint.activate([
            self.view.centerXAnchor.constraint(equalTo: testView.centerXAnchor),
            self.view.centerYAnchor.constraint(equalTo: testView.centerYAnchor),
            testView.heightAnchor.constraint(equalToConstant: 50),
            testView.widthAnchor.constraint(equalToConstant: 200)
        ])
    }
}