Does keyboardVisibilityController fire event when keyboard starts showing or on keyboard is fully shown?

[PavelTurk]

I have a CustomScrollView which I need to scroll down when keyboard is shown. To detect when keyboard is shown I use the following code:

  late StreamSubscription<bool> keyboardSubscription;

  @override
  void initState() {
    super.initState();
    scrollController = new ScrollController();
    var keyboardVisibilityController = KeyboardVisibilityController();
    keyboardSubscription = keyboardVisibilityController.onChange.listen((bool visible) {
      if (visible) {
        Future.delayed(const Duration(milliseconds: 350), () {
          this.scrollToBottom();// here I scroll my CustomScrollView
        });
      }
    });
  }

As it seen I call this.scrollToBottom() with 350 ms delay because otherwise my CustomScrollView is not scrolled. So the question, when is keyboardVisibilityController.onChange.listen called? When keyboard is just starting showing or when keyboard is fully shown and ready for typing? Or could anyone explain why I need to use delay in my code?