Switching to custom keyboard size glitches

I've created a custom keyboard and implemented the:

class KeyboardViewController: UIInputViewController

The imlementation looks like this:

    override func viewDidLoad() {
        super.viewDidLoad()

        var stack = UIStackView()
        stack.axis = .vertical
        stack.spacing = 8
        stack.translatesAutoresizingMaskIntoConstraints = false
        stack.distribution = .fill
        stack.heightAnchor.constraint(equalToConstant: 200).isActive = true
        
        ....

        view.addSubview(stack)
        NSLayoutConstraint.activate([
            stack.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
            stack.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -20),
            stack.topAnchor.constraint(equalTo: view.topAnchor, constant: 10),
            stack.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -10)
        ])

The problem is that the keyboard seems to start showing in the size (I've printed the parent frame):

Optional(<UIView: 0x101008480; frame = (0 0; 390 844); autoresize = W+H; layer = <CALayer: 0x600000207b80>>)

and than resizes to my given height. But it's not fast enough so that I can see some glitches whenever I switch from another keyboard to my custom keyboard.

Is there a way to prevent this resizing or start the keyboard in a given size? This is just not the best user experience.