ListBox with Virtualization Causes InvalidOperationException and UI Freezing in MaterialDesignThemes 5.2.0

[keeleycenc]

Bug explanation

Bug Report: Issue with MaterialDesignThemes Version 5.2.0

Environment:

• MaterialDesignThemes Version: 5.2.0
• WPF: .NET 8.0
• OS: Windows 11 Professional 23H2 22631.4751
• CPU: Intel(R) Core(TM) i7-14700KF 3.40 GHz
• RAM: 32.0 GB

Description

After upgrading to MaterialDesignThemes version 5.2.0 from 5.1.0, the following issues have occurred:

1. UI Freezing/Blocking: The application experiences a brief UI freezing when auto-scrolling and removing items from the ListBox that uses virtualization.
2. System.InvalidOperationException: The following exception occurs intermittently when auto-scrolling a ListBox:

System.InvalidOperationException: "Cannot call StartAt when content generation is in progress."

Steps to Reproduce

1. Upgrade MaterialDesignThemes to version 5.2.0.
2. Use a ListBox with virtualization enabled (e.g., VirtualizingStackPanel.VirtualizationMode="Recycling").
3. Bind a collection (such as InstantMessage) to the ItemsSource of the ListBox.
4. Implement auto-scrolling when new messages are added (e.g., via ScrollIntoView in the ViewModel_OnMessageUpdated method).
5. Remove items from the ListBox (e.g., by updating the collection).
6. Experience brief UI freezing and occasional System.InvalidOperationException.

Code Example

<ListBox
 Height="300"                    
 x:Name="MyInstantMessageListBox"
 ItemContainerStyle="{StaticResource CustomListBoxItemStyle}"
 Template="{StaticResource CustomListBoxTemplate}"
 Grid.Row="1" Grid.ColumnSpan="2"
 Background="Transparent"
 ItemsSource="{Binding InstantMessage}"
 VirtualizingStackPanel.IsVirtualizing="True"
 VirtualizingStackPanel.VirtualizationMode="Recycling">
 <ListBox.ItemsPanel>
     <ItemsPanelTemplate>
         <VirtualizingStackPanel />
     </ItemsPanelTemplate>
 </ListBox.ItemsPanel>
 <ListBox.ItemTemplate>
     <DataTemplate>
         <StackPanel 
             Orientation="Horizontal" 
             VerticalAlignment="Center">
             <TextBlock 
                 Foreground="{DynamicResource TextFillColorPrimaryBrush}"
                 Text="{Binding Timestamp, StringFormat='HH:mm:ss.fff'}" 
                 VerticalAlignment="Center" Width="95"/>
             <md:PackIcon
                 Margin="0, 0, 5, 0"
                 Kind="{Binding Level, Converter={StaticResource InfoLevelToIconKindConverter}}" 
                 Foreground="{Binding Level, Converter={StaticResource InfoLevelToIconColorConverter}}"
                 VerticalAlignment="Center"/>
             <TextBox 
                 Text="{Binding Message}" VerticalAlignment="Center"
                 IsReadOnly="True" BorderThickness="0" Background="Transparent" TextWrapping="Wrap"
                 TextAlignment="Left" HorizontalAlignment="Stretch"/>
         </StackPanel>
     </DataTemplate>
 </ListBox.ItemTemplate>
 <ListBox.Resources>
     <ResourceDictionary>
         <ResourceDictionary.MergedDictionaries>
             <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesign2.Defaults.xaml" />
         </ResourceDictionary.MergedDictionaries>
         <Style TargetType="ListBox">
             <Setter Property="BorderBrush" Value="{DynamicResource MasBorderBrush}"/>
             <Setter Property="BorderThickness" Value="0,0,0,1"/>
         </Style>
     </ResourceDictionary>
 </ListBox.Resources>
</ListBox>

private void ViewModel_OnMessageUpdated(object? sender, EventArgs e) {
    try {
        if (_isMsgAutoScroll && InstantMessageListBox.Items.Count > 0) {
            InstantMessageListBox.ScrollIntoView(InstantMessageListBox.Items[^1]);
        }
    } catch {
        // BUG: ListBox virtualization mode + auto-scrolling error: System.InvalidOperationException:“Cannot call StartAt when content generation is in progress.”
    }
}

Expected Behavior

The application should smoothly auto-scroll the ListBox when new items are added, even if items are removed simultaneously, without causing UI freezing or throwing exceptions.

Actual Behavior

• UI Freezing/Blocking: The UI experiences brief freezing when auto-scrolling and removing items from the ListBox with virtualization enabled.
• Exception: The InvalidOperationException is thrown intermittently with the message: "Cannot call StartAt when content generation is in progress."

Additional Notes

• The issue only occurs after upgrading to version 5.2.0.
• The problem does not occur in version 5.1.0, which was working fine with the same code and configuration.
• Using delay scheduling does not resolve the issue.

Application.Current.Dispatcher.BeginInvoke(new Action(() => {
    InstantMessageListBox.ScrollIntoView(InstantMessageListBox.Items[^1]);
}), DispatcherPriority.Background);

• After removing the MaterialDesign2 resource from ListBox.Resources, the issue disappears, Remove the following code:

<ListBox.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesign2.Defaults.xaml" />
        </ResourceDictionary.MergedDictionaries>
        <Style TargetType="ListBox">
            <Setter Property="BorderBrush" Value="{DynamicResource MasBorderBrush}"/>
            <Setter Property="BorderThickness" Value="0,0,0,1"/>
        </Style>
    </ResourceDictionary>
</ListBox.Resources>

• Queue Updates Every Second: The queue updates every second, with a maximum of 15 items per cycle. The performance is not an issue, and this points to a potential issue with MaterialDesignThemes, rather than the performance of the application.

Screenshot

Version

5.2.0