The position of ComboBox dropdown is incorrect when set it as a child of Popup with DropShadowEfffect. by WinkingZhang

I am sure it is a bug but as well as a rare case, when I edit template for ComboBox, and replace its Popup part with following content:
<Popup x:Name="Popup">
    <Border x:Name="PopupBorder"
        BorderBrush="{TemplateBinding BorderBrush}"
        BorderThickness="{TemplateBinding BorderThickness}"
        CornerRadius="3"
        HorizontalAlignment="Stretch"
        Height="Auto">
        <Border.Effect>
            <DropShadowEffect/>
        </Border.Effect>
        <Border.Background>
            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                <GradientStop Color="#FFFFFFFF" Offset="0"/>
                <GradientStop Color="#FFFEFEFE" Offset="1"/>
            </LinearGradientBrush>
        </Border.Background>
        <!--<ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1">
            <ItemsPresenter/>
        </ScrollViewer>-->
        <ComboBox x:Name="childCombo"/>
    </Border>
</Popup>
OK, I just wanna add DropShadowEffect for dropdown, and the dropped content is templated with a ComboBox.
When running the application, after click the dropped ComboBox, the popup position is incorrect, it's on left top, but not near the ComboBox.