Border does not show for expander in popup - wpf

Question

Not sure what I'm doing wrong: but for some reason I cannot make the Expander control show its border even though I have specified it in the XAML.
Can someone help me get the border to show up.
Here is the XAML:
<Popup x:Name="PopLink" StaysOpen="True" Placement="Mouse" PopupAnimation="Fade" AllowsTransparency="True" MaxWidth="300">
<Popup.Style>
<Style TargetType="Popup">
<EventSetter Event="MouseLeave" Handler="OnPopupMouseLeave"></EventSetter>
</Style>
</Popup.Style>
<Border BorderBrush="{DynamicResource Esri_BorderBrush}" BorderThickness="2" Background="{DynamicResource Esri_ControlBackgroundBrush}">
<StackPanel>
<DockPanel>
<TextBlock Margin="10 10 5 0" Text="{Binding PopupDesc}" FontWeight="Bold" HorizontalAlignment="Left"></TextBlock>
<Button BorderThickness="0" Margin="0 -1 0 0 " x:Name="ButtonClosePopup" Click="ButtonClosePopup_OnClick" Width="15" Height="20" HorizontalAlignment="Right" Content="X"></Button>
</DockPanel>
<Separator Margin="10 10 10 5"/>
<TextBlock Margin="10 3 10 5" TextWrapping="Wrap" FontWeight="Bold" FontSize="14" Text="{Binding PopupTitle}"></TextBlock>
<TextBlock Margin="10 3 10 5" TextWrapping="Wrap" FontStyle="Italic" Text="{Binding PopupAuthors}"></TextBlock>
<TextBlock Margin="10 5 10 0" TextWrapping="Wrap" Text="{Binding PopupText}"></TextBlock>
<Expander BorderBrush="DarkGray" BorderThickness="1" Visibility="{Binding IsPopupAbstractVisible}" Header="Abstract" Margin="10 5" >
<TextBlock Height="100" TextWrapping="Wrap">
<TextBox TextWrapping="Wrap" MaxHeight="150" Margin="5" Text="{Binding Path=PopupAbstract}" VerticalScrollBarVisibility="Visible" BorderThickness="0"></TextBox>
</TextBlock>
</Expander>
<TextBlock Margin="10 0 10 10" TextWrapping="Wrap" Text="{Binding PopupKeywords}" FontStyle="Italic"></TextBlock>
</StackPanel>
</Border>
</Popup>