Silverlight 4 Expander/ScrollViewer scrolls down to show last item. How to prevent it?

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C#

Silverlight

Using Silverlight 4, VS 2010 and Blend 4

I have an Expander with a StackPanel and a ItemsControl where I get the collection of items I wanto to display.

<toolkit:Expander x:Name="ExpanderComentarios" Header="{Binding mySugest_Conteudo.Qt_CommentsSugestao_Info, Converter={StaticResource DescComentariosConverter}, ElementName=userControl, Mode=OneWay}" BorderThickness="0" Style="{StaticResource StyleExpanderBlue4}" d:LayoutOverrides="Height" Margin="3">
   <StackPanel x:Name="stackMySugest" Margin="0,3,0,0">
      <ItemsControl x:Name="itemsMySugest" ItemsSource="{Binding mySugest_Conteudo.ListMyComment_Info, ElementName=userControl}">
         <ItemsControl.ItemTemplate>
            <DataTemplate>
               <componente:myComent x:Name="mySugest_Comments" myComment="{Binding}"/>
            </DataTemplate>
         </ItemsControl.ItemTemplate>
      </ItemsControl>
   </StackPanel>
</toolkit:Expander>

The item collection has a few items, more than would fit on the screen.

This Expander is inside a ScrollViewer.

The thing is that whenever I expand the Expander or update its content the ScrollViewer scrolls down to show the last item of the Expander.

How can I prevent that scroll from hapening?