start scrolling when the UITextView hits a certain number of lines? swift 4

Question

So I'm making a messaging app, and I have an inputAccessoryView called typingView, which contains a messageTextView: UITextView and sendButton: UIButton. I have a separate class that is called class InputAccessoryView: UIView, UITextViewDelegate. This is my code for setting up the inputAccessoryView:

var typingView: InputAccessoryView!

lazy var typingViewContainer: UIView = {
    typingView = InputAccessoryView(frame: CGRect(x: 0, y: 0, width: self.view.frame.size.width, height: fakeTypingViewHeight.constant))
    return typingView
}()

override var inputAccessoryView: UIView? {
    get {
        return typingViewContainer
    }
}

override var canBecomeFirstResponder: Bool {
    get {
        return true
    }
}

Then I add an observer to act on the event that the contentSize of the UITextView changes:

messageTextView.addObserver(self, forKeyPath: "contentSize", options: .new, context: nil)

I handle this correctly - every time the contentSize changes by x lines, I increase the height of the typingView by the height of x lines by calling typingView.invalidateIntrinsicContentSize(). Something like this.

Here is my code for recalculating the size:

override var intrinsicContentSize: CGSize {
    let sizeToFitIn = CGSize(width: messageTextView.bounds.size.width, height: .greatestFiniteMagnitude)
    let newSize = messageTextView.sizeThatFits(sizeToFitIn)
    let newHeight = newSize.height
    return CGSize(width: bounds.width, height: newHeight)
}

Also, messageTextView.isScrollEnabled = false, because of this post here.

So it recalculates the height correctly every time the contentSize changes, but my question is: If I want the UITextView to start scrolling at say, 5 lines, how would I do that?