Silverlight 4 - ScrollViewer and child DataGrid MinHeight

edit: I'm rewriting almost the entire question because I realized the question was incorrect and confusing. I apologize for this, but the question had incorrect assumptions that made it impossible to answer. I originally tried to simplify it to make it easier to understand, but this made it impossible to replicate my problem.

If I have an DataGrid with a MinHeight in a ScrollViewer, I would expect that as my ViewPort shrinks, the ActualHeight of the element would be decreased until it hits MinHeight before the scrollbars show up.

Instead, it seems that when the datagrid's rows cumulative heights add up to more than the MinHeight, this value overrides MinHeight

Is there a way to do this without manually sizing everything and having a ton of code?

Example:

<ScrollViewer VerticalScrollBarVisibility="Auto" Background="Red">
    <Grid x:Name="LayoutRoot" Background="Black" HorizontalAlignment="Stretch">
        <Grid.RowDefinitions>
            <RowDefinition MinHeight="20"/>
            <RowDefinition Height="80"/>
        </Grid.RowDefinitions>

        <sdk:DataGrid AutoGenerateColumns="True" Name="dataGrid1" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" MinHeight="20" />
        <Rectangle Fill="Blue" Width="100" Height="80" Grid.Row="1" />
    </Grid>
</ScrollViewer>

If you were to populate this grid with some rows, if you maximize the window, the grid takes up most of the space and has white space after the rows. If you shrink it down, the layout takes away from the white space until that space runs out, then the root level ScrollViewer kicks in, even though MinHeight has not been reached.

If you replace the DataGrid with another rectangle, the behavior is different (obviously). The new rectangle would shrink down to height 20.

How do I achieve this with the grid? My requirements are to have nested scrollbars on my SL page (which I find distasteful, but it's not in my control). The idea is that the top level scrollbars are a "last resort" of sorts.

What about this:

<ScrollViewer>
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
        <RowDefinition Height="250" />
    </Grid.RowDefinitions>      
    <Rectangle MinHeight="150" Background="Red" Grid.Row="0" />
    <Rectangle Height="250" Background="Blue" Grid.Row="1" />
</Grid>

You did not have the Grid.Row values set on either of the rectangles.

You've not provided sufficient information to solve your specific problem. However, it is easy to demonstrate that the ScrollViewer does work in exactly the fashion you desire by distilling down to something as simple as:

<UserControl ...>
    <ScrollViewer VerticalScrollBarVisibility="Auto">
        <Border MinHeight="200" BorderBrush="Blue" BorderThickness="1" Background="Red"/>
    </ScrollViewer>
</UserControl>

Put this in a standalone Silverlight application in the main page and you'll see that the ScrollViewer only displays the vertical scroll bar when the window is small enough. You can download the solution here.

This is because ScrollViewer itself has a border and padding that occupies little space of its own. Try considering little extra height that should match space of scrollbar border.

Another option will be to change the control template of scrollviewer and remove the border and extra space occupied around content presenter. And set horizontal scroll visibility to collapsed so it will not occupy space.