title: DataGrid can't scroll b/c drag drop
body: <p>I have a <code>DataGrid</code> and has implemented with drag drop features 
  Now when I try to scroll, it does not work simply display a Nude sign. 
  I subscribe <code>ScrollBar.Scroll</code>, but inside it, I can't get the info how to scroll, 
  I do not know if I should scroll up or down. I must have missed something. Pls help. thanks </p>
  
  <pre><code>&lt;toolkit:DataGrid x:Name="dg"                                                                  
      Style="{StaticResource DataGridStyle}"
      RowDetailsVisibilityMode="VisibleWhenSelected" 
      ItemsSource="{Binding Relations}"                           
      SelectionUnit="FullRow" 
      SelectedItem="{Binding ListSelection}" IsReadOnly="True" 
      VerticalScrollBarVisibility="Auto" 
      HorizontalScrollBarVisibility="Auto"
      MouseDoubleClick="dg_MouseDoubleClick"
      PreviewKeyDown="DgPreviewKeyDown"
      PreviewMouseWheel="DgPreviewMouseWheel"
      PreviewMouseLeftButtonDown="DataGridPreviewMouseLeftButtonDown"
      PreviewMouseMove="DataGridMouseMove"
      Drop="DataGridDrop" 
      DragEnter="DataGridDragEnter"
      ScrollBar.Scroll="DgScroll"&gt;
      &lt;toolkit:DataGrid.Columns&gt;
          &lt;toolkit:DataGridTextColumn Header="Symbol" Binding="{Binding Relation}" MinWidth="310"&gt;
              &lt;toolkit:DataGridTextColumn.ElementStyle&gt;
                  &lt;Style TargetType="{x:Type TextBlock}"&gt;
                      &lt;Setter Property="TextWrapping" Value="Wrap" /&gt;
                  &lt;/Style&gt;
              &lt;/toolkit:DataGridTextColumn.ElementStyle&gt;
          &lt;/toolkit:DataGridTextColumn&gt;
          &lt;toolkit:DataGridTextColumn Header="Description" Width="*" MinWidth="300" Binding="{Binding Description}"&gt;
              &lt;toolkit:DataGridTextColumn.ElementStyle&gt;
                  &lt;Style TargetType="{x:Type TextBlock}"&gt;
                      &lt;Setter Property="TextWrapping" Value="Wrap" /&gt;
                  &lt;/Style&gt;
              &lt;/toolkit:DataGridTextColumn.ElementStyle&gt;
          &lt;/toolkit:DataGridTextColumn&gt;
      &lt;/toolkit:DataGrid.Columns&gt;
  &lt;/toolkit:DataGrid&gt;
  
  private void DgScroll(object sender, ScrollEventArgs e)
  {
      var viewer = DragDropHelper.GetDescendantByType(dg, typeof(ScrollViewer)) as ScrollViewer;
      if (viewer == null) return;
  
      var newVal = e.NewValue;  //How I can know scroll up or down???
      viewer.ScrollToVerticalOffset(viewer.VerticalOffset + 30)       
  }
  </code></pre>