WPF ListBox virtualization screws up displayed items

Question

Problem

We need to efficiently display a large (>1000) number of objects in a WPF ListBox control. We are relying on the WPF ListBox’s virtualization (via VirtualizingStackPanel) to efficiently display these items.

Bug: The WPF ListBox control does not display items correctly when using virtualization.

How to Reproduce

We have distilled the problem to the standalone xaml shown below.

Copy and paste the xaml into XAMLPad.

Initially, there is no selected item in the ListBox, so as expected, all items are the same size and they completely fill the available space.

Now, click on the first item. As expected, because of our DataTemplate, the selected item will expand to show additional information.

As expected, this causes the horizontal scrollbar to appear, since the selected item is now wider than the available space.

Now use the mouse to click and drag the horizontal scrollbar to the right.

Bug: the non-selected visible items no longer stretch to fill the available space. All the visible items should be the same width.

Is this a known bug? Is there any way to fix this, either via XAML or programmatically?

---

<Page 
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:sys="clr-namespace:System;assembly=mscorlib" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
    <Page.Resources>

        <DataTemplate x:Key="MyGroupItemTemplate">
            <Border Background="White"
                    TextElement.Foreground="Black"
                    BorderThickness="1"
                    BorderBrush="Black"
                    CornerRadius="10,10,10,10"
                    Cursor="Hand"
                    Padding="5,5,5,5"
                    Margin="2"
                    >
                <StackPanel>
                    <TextBlock Text="{Binding Path=Text, FallbackValue=[Content]}" />
                    <TextBlock x:Name="_details" Visibility="Collapsed" Margin="0,10,0,10" Text="[xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]" />
                </StackPanel>
            </Border>
            <DataTemplate.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type ListBoxItem}},Path=IsSelected}"
                             Value="True">
                    <Setter Property="TextElement.FontWeight"
                            TargetName="_details"
                            Value="Bold"/>
                    <Setter Property="Visibility"
                            TargetName="_details"
                            Value="Visible"/>
                </DataTrigger>
            </DataTemplate.Triggers>
        </DataTemplate>

    </Page.Resources>

    <DockPanel x:Name="LayoutRoot">

        <Slider x:Name="_slider"
                DockPanel.Dock="Bottom" 
                Value="{Binding FontSize, ElementName=_list, Mode=TwoWay}" 
                Maximum="100"
                ToolTip="Font Size"
                AutoToolTipPlacement="BottomRight"/>

        <!--
          I want the items in this ListBox to completly fill the available space.
          Therefore, I set HorizontalContentAlignment="Stretch".

          By default, the WPF ListBox control uses a VirtualizingStackPanel.
          This makes it possible to view large numbers of items efficiently.
          You can turn on/off this feature by setting the ScrollViewer.CanContentScroll to "True"/"False".

          Bug: when virtualization is enabled (ScrollViewer.CanContentScroll="True"), the unselected
               ListBox items will no longer stretch to fill the available horizontal space.
               The only workaround is to disable virtualization (ScrollViewer.CanContentScroll="False").
        -->

        <ListBox x:Name="_list"
                 ScrollViewer.CanContentScroll="True"
                 Background="Gray" 
                 Foreground="White"
                 IsSynchronizedWithCurrentItem="True" 
                 TextElement.FontSize="28"
                 HorizontalContentAlignment="Stretch"
                 ItemTemplate="{DynamicResource MyGroupItemTemplate}">
            <TextBlock Text="[1] This is item 1." />
            <TextBlock Text="[2] This is item 2." />
            <TextBlock Text="[3] This is item 3." />
            <TextBlock Text="[4] This is item 4." />
            <TextBlock Text="[5] This is item 5." />
            <TextBlock Text="[6] This is item 6." />
            <TextBlock Text="[7] This is item 7." />
            <TextBlock Text="[8] This is item 8." />
            <TextBlock Text="[9] This is item 9." />
            <TextBlock Text="[10] This is item 10." />
        </ListBox>

    </DockPanel>
</Page>

Answer 1

I spent more time attempting this than I probably should have, and couldn't get it to work. I understand what's going on here, but in pure XAML, I'm having trouble figuring out how to solve the issue. I think I see how to solve the problem, but it involves a converter.

Warning: Things are going to get complicated as I explain my conclusions.

The underlying problem comes from the fact that the Width of the controls stretch to the Width of their container. When virtualization is enabled, the Width will not change. In the underlying ScrollViewer inside of ListBox, the ViewportWidth property corresponds to the Width you see. When another control stretches out further (you select it), the ViewportWidth is still the same, but the ExtentWidth shows the full width. Binding the width of all controls to that of the ExtentWidth should work...

But it doesn't. I set the FontSize to 100 for quicker testing in my case. When an item is selected, ExtentWidth="4109.13. Going down the tree to your ControlTemplate's Border, I see ActualWidth="4107.13". Why the 2 pixel difference? ListBoxItem contains a Border with 2 Pixel padding, causing the ContentPresenter to render slightly smaller.

I added the following Style with help from here to allow me to directly access the ExtentWidth:

<Style x:Key="{x:Type ListBox}" TargetType="ListBox">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="ListBox">
        <Border 
          Name="Border" 
          Background="White"
          BorderBrush="Black"
          BorderThickness="1"
          CornerRadius="2">
          <ScrollViewer 
            Name="scrollViewer"
            Margin="0"
            Focusable="false">
            <StackPanel IsItemsHost="True" />
          </ScrollViewer>
        </Border>
        <ControlTemplate.Triggers>
          <Trigger Property="IsEnabled" Value="false">
            <Setter TargetName="Border" Property="Background"
                    Value="White" />
            <Setter TargetName="Border" Property="BorderBrush"
                    Value="Black" />
          </Trigger>
          <Trigger Property="IsGrouping" Value="true">
            <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
          </Trigger>
        </ControlTemplate.Triggers>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

Note I added a name to ScrollViewer for this purpose.

Then, I attempted to bind the Width of your Border to the ExtentWidth:

Width="{Binding ElementName=scrollViewer, Path=ExtentWidth}"

However, because of that 2 pixel padding, the controls will resize in an infinite loop, with the padding adding 2 pixels to the ExtentWidth, which resizes the Border width, which adds 2 more pixels to the ExtentWidth, etc. until you delete the code and refresh.

If you added a Converter that subtracted 2 from the ExtentWidth, I think this might work. However, when the scroll bar does not exist (you have not selected anything), ExtentWidth="0". Thus, binding to MinWidth instead of Width may work better so the items appear correctly when no scroll bar is visible:

MinWidth="{Binding ElementName=scrollViewer, Path=ExtentWidth, Converter={StaticResource PaddingSubtractor}}"

A better solution would be if you could directly databind the MinWidth of the ListBoxItem itself. You could bind directly to ExtentWidth, and no converter would be necessary. However I have no idea how to get access to that item.

Edit: For organization sake, here's the clip required to do that. Makes everything else unnecessary:

<Style TargetType="{x:Type ListBoxItem}">
    <Setter Property="MinWidth" Value="{Binding Path=ExtentWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ScrollViewer}}}" />
</Style>

Answer 2

Thanks to Will's great analysis!

Based on Will's suggestion: "A better solution would be if you could directly databind the MinWidth of the ListBoxItem itself...However I have no idea how to get access to that item", I was able to implement that using pure xaml, as follows:

<ListBox x:Name="_list" 
         Background="Gray" 
         Foreground="White"
         IsSynchronizedWithCurrentItem="True" 
         TextElement.FontSize="28"
         HorizontalContentAlignment="Stretch"
         ItemTemplate="{DynamicResource MyGroupItemTemplate}">

    <!-- Here is Will's suggestion, implemented in pure xaml. Seems to work.
         Next problem is if you drag the Slider to the right to increase the FontSize.
         This will make the horizontal scroll bar appear, as expected.
         Problem: the horizontal scroll bar never goes away if you drag the Slider to the left to reduce the FontSize.
    -->
    <ListBox.Resources>
        <Style TargetType="{x:Type ListBoxItem}">
            <Setter Property="MinWidth" Value="{Binding Path=ExtentWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ScrollViewer}}}" />
        </Style>
    </ListBox.Resources>

    <TextBlock Text="[1] This is item 1." />
    <TextBlock Text="[2] This is item 2." />
    <TextBlock Text="[3] This is item 3." />
    <TextBlock Text="[4] This is item 4." />
    <TextBlock Text="[5] This is item 5." />
    <TextBlock Text="[6] This is item 6." />
    <TextBlock Text="[7] This is item 7." />
    <TextBlock Text="[8] This is item 8." />
    <TextBlock Text="[9] This is item 9." />
    <TextBlock Text="[10] This is item 10." />
</ListBox>

I got the idea from Adam Nathan's great book, "Windows Presentation Foundation Unleashed".

So, this seems to fix the original problem.

New Problem

You notice that there is a Slider control in the xaml that let's you increase/decrease the ListBox font. The idea here was to allow the user the ability to scale the ListBox content up or down for easier visibility.

If you first drag the Slider to the right to increase the FontSize, this will make the horizontal scroll bar appear, as expected. The new problem is that the horizontal scroll bar never goes away if you drag the Slider to the left to reduce the FontSize.

Any ideas?