WPF : Combo-box Scrolling by MouseWheel works, but scroll bar clicking does not work

I am stuck in the strange situation with WPF Combo-box, Scrolling by Mouse wheel is perfectly working, But when clicked on mouse down arrow of Scrollbar, Items hide, Same happens when trying to drag the scroll bar.

I am using below XAML code

<ComboBox  Name="cmbPageNumber" >
     <ComboBox.ItemContainerStyle>
         <Style TargetType="ComboBoxItem">
               <EventSetter Event="PreviewMouseDown" Handler="item_PreviewMouseDown"/>
         </Style>
     </ComboBox.ItemContainerStyle>
</ComboBox>    

  void item_PreviewMouseDown(object sender, MouseButtonEventArgs e)
    {
        ComboBoxItem cmbitem = sender as ComboBoxItem;
        cmbitem.IsSelected = true;
        string value = cmbitem.Content.ToString();

        OVTreeItem item = CurrentTreeItemForPaging;
        if (item != null && item.PageCounts >= Convert.ToInt32(value))
            item.GotoPage = Convert.ToInt32(value);

        PagingInfo(item);
        EnableDisablePagingButtons();
        this.cmbPageNumber.IsDropDownOpen = false;
        e.Handled = true;
    }

To populate combobox, I am using below code

       for (int i = 1; i <= item.PageCounts; i++)
            {
                cmbPageNumber.Items.Add(i);

            }

The interesting thing, When I do right click on combo-box and reopen again, then mouse click and scroll by drag working.

Please help me.