Setting UIScrollView's contentInset for keyboardWillShowNotification not working properly

I have a ViewController with the following structure ((x) indicating the level):

UIViewController    (1)
  - NavigationBar   (2)
  - UIScrollView    (2)
    - UIView        (3)
      - UITextField (4)
      - UITextField (4)
      - UITextField (4)
      - UITextField (4)
      - UIButton    (4)

All elements of level 4 are vertically constrained to eachother with a spacing of 16.
The first and last elements of level 4 are constrained to the UIView's (3) top and bottom.
The UIView (3) is constrained with top and bottom to the UIScrollView (2).
The UIScrollView (2) is constrained to the NavigationBar's bottom (2) and the superview's bottom (1)
(Of course there are the necessary horizontal constraints as well!)

The UIView (3) has the following constraints:

Leading constraint of 0 to all subViews.
Trailing constraint of 0 to all subviews.
Bottom space of 24 to UIButton (should add some extra spacing)
Top space of 24 to the topmost UITextField (top spacing)
Top space of 0 to superView (UIScrollView)
Bottom space of 0 to superView (UIScrollView)
'Equal width - minus 32' to NavigationBar (so — fixed width)

In the viewDidLoad of the viewController, I call this:

registerForKeyboardWillShowNotification(self.scrollView)
registerForKeyboardWillHideNotification(self.scrollView)

Where registerForKeyboard...ShowNotification is an extension of UIViewController:

extension UIViewController
{
    /// Act when keyboard is shown, by adding contentInsets to the scrollView.
    func registerForKeyboardWillShowNotification(_ scrollView: UIScrollView, usingBlock block: ((CGSize?) -> Void)? = nil)
    {
        _ = NotificationCenter.default.addObserver(forName: UIResponder.keyboardWillShowNotification,
                                                   object: nil, queue: nil)
        { notification in
            let userInfo      = notification.userInfo!
            let keyboardSize  = (userInfo[UIResponder.keyboardFrameEndUserInfoKey]! as AnyObject).cgRectValue.size
            let contentInsets = UIEdgeInsets(top: scrollView.contentInset.top,
                                             left: scrollView.contentInset.left,
                                             bottom: keyboardSize.height,
                                             right: scrollView.contentInset.right)

            scrollView.contentInset = contentInsets
            block?(keyboardSize)
        }
    }

    /// Act when keyboard is hidden, by removing contentInsets from the scrollView.
    func registerForKeyboardWillHideNotification(_ scrollView: UIScrollView, usingBlock block: ((CGSize?) -> Void)? = nil)
    {
        _ = NotificationCenter.default.addObserver(forName: UIResponder.keyboardWillHideNotification,
                                                   object: nil, queue: nil)
        { notification in
            let userInfo = notification.userInfo!
            let keyboardSize = (userInfo[UIResponder.keyboardFrameEndUserInfoKey]! as AnyObject).cgRectValue.size
            let contentInsets = UIEdgeInsets(top: scrollView.contentInset.top,
                                             left: scrollView.contentInset.left,
                                             bottom: 0,
                                             right: scrollView.contentInset.right)

            scrollView.contentInset = contentInsets
            block?(keyboardSize)
        }
    }
}

However, when the keyboard shows, it doesn't inset the scrollView (enough). I debugged, and this is the case:

Regular keyboard: height = 216
Regular keyboard with suggestion bar: height = 260
iPhone X keyboard with suggestion bar: height = 291

I first though the suggestion bar could be the problem, but it's not.

In registerForKeyboardWillShowNotification I changed bottom: keyboardSize.height to bottom: keyboardSize.height + 30, which gives exactly the same result (I see the same part of the button that's partially hidden behind the keyboard). Once I add 50 or more, it finally seems to make a small difference.

Instead of keyboardWillShowNotification I tried keyboardDidShowNotification, this doesn't make a difference.
Instead of keyboardFrameEndUserInfoKey I tried keyboardFrameBeginUserInfoKey, this doesn't make a difference.

What am I missing here?

Solution

Unfortunately I haven't been able to solve this, I'm unsure why this doesn't work as expected.

However, I got the expected behaviour by using a UIStackView inside the UIScrollView. I used this article as reference.

The UI layout

UIViewController (1)
- NavigationBar (2)
- UIScrollView (2)
- UIStackView (3)
  - UITextField (4)
  - UITextField (4)
  - UITextField (4)
  - UITextField (4)
  - UIButton (4)

The UIScrollView

ScrollView's leading and trailing are constrained with 16 to the superView.
ScrollView's top is constrained with 0 to the navigationBar's bottom.
ScrollView's bottom is constrained with 0 to the superView's bottom.

The UIStackView

StackView's leading and trailing are constrained with 0 to the scrollView.
StackView has an equal width to scrollView.
StackView's top and bottom are constrained with 24 to the scrollView, to get the desired spacing to the navigationBar, and between the button and the keyboard.
StackView is setup as axis=vertical, alignment=fill, distribution=fill, spacing=24.

The NavigationBar

The NavigationBar is a custom class that derives its height from its contents. This didn't have a height constraint set, but a placeholder height of 100. The NavigationBar would fill the entire screen. This is solved by removing the placeholder height, and adding any height constraint with a low priority (in this case a priority of 1).

The initial code for applying the keyboard inset works now.

/// Act when keyboard is shown, by adding contentInsets to the scrollView.
func registerForKeyboardWillShowNotification(_ scrollView: UIScrollView, usingBlock block: ((CGSize?) -> Void)? = nil)
{
    _ = NotificationCenter.default.addObserver(forName: UIResponder.keyboardWillShowNotification,
                                               object: nil, queue: nil)
    { notification in
        let userInfo      = notification.userInfo!
        let keyboardSize  = (userInfo[UIResponder.keyboardFrameEndUserInfoKey]! as AnyObject).cgRectValue.size
        let contentInsets = UIEdgeInsets(top: scrollView.contentInset.top,
                                         left: scrollView.contentInset.left,
                                         bottom: keyboardSize.height,
                                         right: scrollView.contentInset.right)

        scrollView.contentInset = contentInsets
        block?(keyboardSize)
    }
}