Content layout incorrectly styled inside a ToolWindow

Hi,

The following XAML fragment is from a UserControl loaded into a ToolWindow at runtime. When displayed within the ToolWindow the UniformGrid row sizing is inconsistent. But on copying to a vanillla WPF application all rows are the same height.

<ScrollViewer VerticalScrollBarVisibility="Auto">
<ItemsControl Height="Auto"
ItemsSource="{Binding Path=Parameters[0].Bits}"
Background="Transparent">

<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<UniformGrid Background="White" Rows="{Binding Items.Count, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ItemsControl}}}" IsItemsHost="True" Columns="1">

</UniformGrid>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>

<ItemsControl.ItemTemplate>
<DataTemplate>
<Border BorderThickness="1" BorderBrush="Black">
<Rectangle Fill="Blue"></Rectangle>
</Border>

</DataTemplate>
</ItemsControl.ItemTemplate>

</ItemsControl>
</ScrollViewer>

Cheers

Mark