WPF DataGrid does not show scrollbars and ends from the visible area

Like others, I have a DataGrid that does not show scrollbars. I find it unique to my situation that I don't see the StackPanel anywhere in the visual or logical tree. I am using WPF Inspector to view trees. I tried various suggestions to set the height and width of the contained columns and rows of the Grid without success. I'm sure something is missing for me, which allows the content to go beyond the visible area, but I can’t say what it is. Any help would be greatly appreciated. This application is a WPF prism with the MEF application, and the DataGrid is located in the UserControl, which is located in the Prism zone.

XAML shell window:

<Window>
  <Grid x:Name="GridOuterShell">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>

    <ribbon:Ribbon Grid.Row="0" >
        ...
    </ribbon:Ribbon>

    <Grid x:Name="GridShellContent" Grid.Row="1">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="350" MinWidth="300"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>


        <local:RegionBorderControl Grid.Row="0" Grid.Column="0" Grid.RowSpan="3" Margin="2,2,8,2" RegionName="{Binding MainRegionDisplayName}"
                               Style="{DynamicResource RegionBorderControlStyle}">
        <ContentControl prism:RegionManager.RegionName="MainRegion"
                        VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch"/>

        </local:RegionBorderControl>


        <GridSplitter Grid.Row="0" Grid.Column="1" Grid.RowSpan="3" HorizontalAlignment="Center" VerticalAlignment="Stretch"
                  Width="3" ShowsPreview="True" ResizeDirection="Columns" />

        <local:RegionBorderControl Grid.Row="0" Grid.Column="2" RegionName="{Binding RightTopRegionDisplayName}"
                               Style="{DynamicResource RegionBorderControlStyle}">
        <ContentControl prism:RegionManager.RegionName="RightTopRegion"
                        VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch"/>

        </local:RegionBorderControl>

        <GridSplitter Grid.Row="1" Grid.Column="3" HorizontalAlignment="Stretch" VerticalAlignment="Center"
                  Height="3" ShowsPreview="true" ResizeDirection="Rows" ResizeBehavior="PreviousAndNext" Background="Silver"/>

        <local:RegionBorderControl Grid.Row="2" Grid.Column="2" RegionName="{Binding RightBottomRegionDisplayName}"
                               Style="{DynamicResource RegionBorderControlStyle}">
            <ContentControl prism:RegionManager.RegionName="RightBottomRegion"/>

        </local:RegionBorderControl>

    </Grid>

    <StatusBar Grid.Row="2">
        ...
    </StatusBar>

  </Grid>
</Window>

UserControl XAML:

<UserControl>

<Grid x:Name="GridMain">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition />
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <DockPanel Grid.Row="0" LastChildFill="False" HorizontalAlignment="Stretch" Width="Auto" >
            <ToolBar x:Name="tbToolBar" DockPanel.Dock="Left" Background="{x:Null}">
                ...
            </ToolBar>
        </DockPanel>

        <DataGrid AutoGenerateColumns="False" Grid.Row="2" Name="DataGridList" ItemsSource="{Binding MyItems}" IsReadOnly="True" CanUserResizeRows="False" SelectionMode="Single" 
                    SelectedItem="{Binding Path=SelectedDataGridRecord, Mode=TwoWay}" Style="{StaticResource DataGridDefault}" >
            <DataGrid.Columns>
                ...
            </DataGrid.Columns>
        </DataGrid>

    </Grid>