屏幕分辨率改变后，WPF Popup不在顶部？

Question

<Border Name="PART_MegaMenuHoverArea" Padding="{TemplateBinding Padding}" CornerRadius="5" BorderThickness="0" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<Grid Panel.ZIndex="1000">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ContentPresenter Grid.Column="0" ContentSource="Header" Margin="0,-3,0,0"/>
<Image x:Name="Checkmark" Grid.Column="1" Source="../Images/Checkmark.png" Visibility="Collapsed" Margin="10,-1,0,0"/>
</Grid>
</Border>
<Popup Name="PART_MegaMenuPopup" AllowsTransparency="True" PlacementTarget="{Binding ElementName=PART_MegaMenuHoverArea}" PlacementRectangle="{TemplateBinding PlacementRectangle}"
StaysOpen="False" Placement="Bottom">

其余的代码是UI逻辑，所以绑定都在前端。它适用于Windows 7，但在Windows XP上，屏幕分辨率更改后，弹出窗口会在主窗口后面呈现。有什么想法吗？