Fluent.ComboBox.OnDropDownOpened C# (CSharp) Method

        protected override void OnDropDownOpened(EventArgs e)
        {
            base.OnDropDownOpened(e);
            Mouse.Capture(this, CaptureMode.SubTree);
            if (SelectedItem != null) Keyboard.Focus(ItemContainerGenerator.ContainerFromItem(SelectedItem) as IInputElement);
            focusedElement = Keyboard.FocusedElement;
            focusedElement.LostKeyboardFocus += OnFocusedElementLostKeyboardFocus;

            canSizeX = true;
            canSizeY = true;

            galleryPanel.Width = double.NaN;
            scrollViewer.Height = double.NaN;

            FrameworkElement popupChild = popup.Child as FrameworkElement;
            scrollViewer.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
            popupChild.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
            popup.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
            double heightDelta = popupChild.DesiredSize.Height - scrollViewer.DesiredSize.Height;

            double initialHeight = Math.Min(RibbonControl.GetControlWorkArea(this).Height * 2 / 3, MaxDropDownHeight);
            if (!double.IsNaN(DropDownHeight)) initialHeight = Math.Min(DropDownHeight, MaxDropDownHeight);
            if (scrollViewer.DesiredSize.Height > initialHeight)
            {
                scrollViewer.Height = initialHeight;
            }
            else initialHeight = scrollViewer.DesiredSize.Height;

            Rect monitor = RibbonControl.GetControlMonitor(this);
            double delta = monitor.Bottom - this.PointToScreen(new Point()).Y - ActualHeight - initialHeight - heightDelta;
            if (delta >= 0) ShowPopupOnTop = false;
            else
            {
                double deltaTop = this.PointToScreen(new Point()).Y - initialHeight - heightDelta - monitor.Top;
                if (deltaTop > delta) ShowPopupOnTop = true;
                else ShowPopupOnTop = false;

                if (deltaTop < 0)
                {
                    delta = Math.Max(Math.Abs(delta), Math.Abs(deltaTop));
                    if (delta > galleryPanel.GetItemSize().Height)
                    {
                        scrollViewer.Height = delta;
                    }
                    else
                    {
                        canSizeY = false;
                        scrollViewer.Height = galleryPanel.GetItemSize().Height;
                    }

                }
            }
            popupChild.UpdateLayout();
        }