SystemBackdrop has no effect on Popup

[whiskhub]

Describe the bug

Setting SystemBackdrop on Popup (e.g. to Acrylic) has no effect, the popup stays transparent.
Regardless of whether ShouldConstrainToRootBounds is set to True or False.

For a Flyout, setting SystemBackdrop works.

Steps to reproduce the bug

Add the following code to a WinUI 3 window, start the app and click the buttons.

<Grid>
    <Grid.Background>
        <LinearGradientBrush StartPoint="0,0" EndPoint="1,1">
            <GradientStop Color="Yellow" Offset="0.0" x:Name="GradientStop1"/>
            <GradientStop Color="Red" Offset="0.25" x:Name="GradientStop2"/>
            <GradientStop Color="Blue" Offset="0.75" x:Name="GradientStop3"/>
            <GradientStop Color="LimeGreen" Offset="1.0" x:Name="GradientStop4"/>
        </LinearGradientBrush>
    </Grid.Background>
    
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center" Spacing="15">

        <Button Content="Open Flyout">
            <Button.Flyout>
                <Flyout ShouldConstrainToRootBounds="False">
                    <Flyout.FlyoutPresenterStyle>
                        <Style TargetType="FlyoutPresenter" BasedOn="{StaticResource DefaultFlyoutPresenterStyle}">
                            <Setter Property="Background" Value="Transparent" />
                        </Style>
                    </Flyout.FlyoutPresenterStyle>
                    <TextBlock Text="This is a texttttttttttttttttttt" TextWrapping="Wrap"/>
                    <Flyout.SystemBackdrop>
                        <DesktopAcrylicBackdrop/>
                    </Flyout.SystemBackdrop>
                </Flyout>
            </Button.Flyout>
        </Button>

        <Button x:Name="myButton" Click="myButton_Click">Open Popup</Button>

        <Popup x:Name="MyPopup" XamlRoot="{x:Bind myButton.XamlRoot, Mode=OneWay}" ShouldConstrainToRootBounds="False" PlacementTarget="{x:Bind myButton}" DesiredPlacement="Bottom">
            <Grid BorderBrush="Red" BorderThickness="1" MaxWidth="100" Padding="10">
                <TextBlock Text="This is a texttttttttttttttttttt" TextWrapping="Wrap"/>
            </Grid>
            <Popup.SystemBackdrop>
                <DesktopAcrylicBackdrop/>
            </Popup.SystemBackdrop>
        </Popup>
    </StackPanel>
</Grid>

private void myButton_Click(object sender, RoutedEventArgs e)
{
    MyPopup.IsOpen = !MyPopup.IsOpen;
}

Expected behavior

Popup has acrylic background if SystemBackdrop is set to DesktopAcrylicBackdrop.

Screenshots

NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response