Changing combox box items on open/close broken

[grokys]

This seems to have got broken by #4855: adding combo box items when the dropdown is opened now causes the dropdown to be measured incorrectly:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow">
    <ComboBox Name="cb" Items="{Binding Items}" SelectedIndex="{Binding SelectedIndex}" VirtualizationMode="Simple"/>
</Window>

public class MainWindow : Window
    {
        public MainWindow()
        {
            this.InitializeComponent();
            this.AttachDevTools();

            var vm = new ViewModel();
            var cb = this.FindControl<ComboBox>("cb");

            cb.GetObservable(ComboBox.IsDropDownOpenProperty).Skip(1).Subscribe(x =>
            {
                if (x)
                {
                    vm.DropDownOpened();
                }
                else
                {
                    vm.DropDownClosed();
                }
            });

            DataContext = vm;
        }

        private void InitializeComponent()
        {
            AvaloniaXamlLoader.Load(this);
        }
    }

    public class ViewModel : ReactiveObject
    {
        private int _selectedIndex = 0;

        public ObservableCollection<string> Items { get; } = new ObservableCollection<string> { "foo" };

        public int SelectedIndex
        {
            get => _selectedIndex;
            set => this.RaiseAndSetIfChanged(ref _selectedIndex, value);
        }

        public void DropDownOpened()
        {
            var items = new[] { "foo", "bar", "baz" };
            Items.Clear();
            foreach (var i in items)
            {
                Items.Add(i);
            }
            SelectedIndex = 1;
        }

        public void DropDownClosed()
        {
            Items.Clear();
            Items.Add("bar");
            SelectedIndex = 0;
        }
    }

cc: @donandren

[grokys]

Fixed by #4895