UWPCommunityToolkit's PrintHelper produce blank output

Question

I have my code below, basically to print out an image.

private async void imageControl_PrintButtonClick(object sender, RoutedEventArgs e)
{
    var createBitmapTask = Task.Run(async () =>
    {
        var stream = await provider.OpenEntryAsRandomAccessStreamAsync(currentImageFile);
        var decoder = await BitmapDecoder.CreateAsync(stream);
        return await decoder.GetSoftwareBitmapAsync(
               BitmapPixelFormat.Bgra8,
               BitmapAlphaMode.Premultiplied);
    });
    var printHelper = new PrintHelper(printPanel);
    printHelper.OnPreviewPagesCreated += PrintHelper_OnPreviewPagesCreated;
    printPanel.Opacity = 1.0;
    var source = new SoftwareBitmapSource();
    var bitmap = await createBitmapTask;
    await source.SetBitmapAsync(bitmap);

    printImage.Source = source;
    printFileName.Text = "Hello";
    printImage.Height = bitmap.PixelHeight;
    printImage.Width = bitmap.PixelWidth;

    await printHelper.ShowPrintUIAsync("ZipPicView - " + currentImageFile.ExtractFilename(), true);
    printPanel.Opacity = 0;
}

private void PrintHelper_OnPreviewPagesCreated(List<FrameworkElement> obj)
{
    ContentDialog dialog = new ContentDialog();
}

However, the print preview shows an empty page. When I print it out, the printer does nothing.

I've tried changing the opacity of the printPanel (which is a Grid object) to non-zero, and the image does display on the screen. Still, the print has nothing on the output page.

I did notice that, on the OnPreviewPagesCreated, its obj parameter has a new object everytime it's called. The first call has one object with both Width and Height as NaN. My guess is because the container has no size, it cannot determine the content size.

Below is the XAML file.

<Page x:Name="page"
    x:Class="ZipPicViewUWP.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ZipPicViewUWP"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:interactivity="using:Microsoft.Xaml.Interactivity"
    xmlns:behaviors="using:Microsoft.Toolkit.Uwp.UI.Animations.Behaviors"
    mc:Ignorable="d" KeyUp="page_KeyUp" Loaded="page_Loaded" SizeChanged="page_SizeChanged">
    <Canvas x:Name="canvas" SizeChanged="canvas_SizeChanged">
        <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" Height="1019" Width="1920">
            <Grid x:Name="printPanel" Opacity="0">
                <StackPanel x:Name="printContent"
                        Margin="0,0"
                        Orientation="Vertical">
                    <TextBlock x:Name="printFileName" />
                    <Image x:Name="printImage"
                       Stretch="Fill" />
                </StackPanel>
            </Grid>
            <SplitView x:Name="splitView" DisplayMode="CompactOverlay" PanePlacement="Left" CompactPaneLength="50" OpenPaneLength="300">
                <SplitView.Content>
                    <GridView x:Name="thumbnailGrid" />
                </SplitView.Content>
                <SplitView.Pane>
                    <Grid Background="{StaticResource SidebarBackground}">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="50" />
                            <RowDefinition Height="*" />
                            <RowDefinition Height="50" />
                        </Grid.RowDefinitions>
                        <StackPanel Orientation="Horizontal" Grid.Row="0">
                            <Button x:Name="subFolderButton" Width="50" Height="50" Background="Transparent" Click="subFolderButton_Click">
                                <SymbolIcon Symbol="List" />
                            </Button>
                            <TextBlock VerticalAlignment="Center" Margin="0,15">Folders</TextBlock>
                        </StackPanel>
                        <ScrollViewer Grid.Row="1">
                            <ListView x:Name="subFolderListCtrl" SelectionChanged="subFolderList_SelectionChanged" DataFetchSize="2" Margin="-10,0,0,0" />
                        </ScrollViewer>
                        <ProgressRing x:Name="thumbProgress" Height="30" Width="30" HorizontalAlignment="Left" VerticalAlignment="Center" Grid.Row="2" Margin="10,0,0,10" />
                    </Grid>
                </SplitView.Pane>
            </SplitView>
            <interactivity:Interaction.Behaviors>
                <behaviors:Blur x:Name="BlurBehavior" Duration="500" AutomaticallyStart="False" />
            </interactivity:Interaction.Behaviors>
        </Grid>
        <Border x:Name="imageBorder" Visibility="Collapsed">
            <Image x:Name="image" ManipulationMode="TranslateX" ManipulationCompleted="image_ManipulationCompleted" Tapped="image_Tapped">
                <interactivity:Interaction.Behaviors>
                    <behaviors:Blur x:Name="ImageTransitionBehavior" Duration="500" AutomaticallyStart="False" />
                </interactivity:Interaction.Behaviors>
            </Image>
        </Border>

        <local:ViewerControl x:Name="imageControl" Visibility="Collapsed" CloseButtonClick="imageControl_CloseButtonClick" NextButtonClick="imageControl_NextButtonClick" PrevButtonClick="imageControl_PrevButtonClick" SaveButtonClick="imageControl_SaveButtonClick" PrintButtonClick="imageControl_PrintButtonClick" />
        <Border x:Name="loadingBorder" Visibility="Collapsed">
            <ProgressRing IsActive="True" Width="100" Height="100" />
        </Border>
    </Canvas>
    <Page.TopAppBar>
        <CommandBar VerticalContentAlignment="Center" VerticalAlignment="Center">
            <CommandBar.Content>
                <TextBlock Margin="10,0,0,0" x:Name="filenameTextBlock" Text="&lt;None&gt;" UseLayoutRounding="True" />
            </CommandBar.Content>
            <AppBarToggleButton x:Name="fullscreenButton" Icon="FullScreen" Label="FullScreen" Checked="fullscreenButton_Checked" Unchecked="fullscreenButton_Unchecked" />
            <AppBarSeparator />
            <AppBarButton x:Name="openFileButton" Icon="OpenFile" Label="Open File" Click="openFileButton_Click" />
            <AppBarButton x:Name="openFolderButton" Icon="Folder" Label="Open Folder" Click="openFolderButton_Click" />
        </CommandBar>
    </Page.TopAppBar>
</Page>

And the source code is here : https://github.com/wutipong/ZipPicViewCS/tree/master/ZipPicViewUWP