DataGrid with Groups and VirtualizingPanel.IsVirtualizingWhenGrouping="True"

Question

I'm encountering a weird behavior with a DataGrid that has a GroupDescription, and with the option VirtualizingPanel.IsVirtualizingWhenGrouping="True" on the xaml.

Here's part of the code:

In the ViewModel:

if (collectionView.GroupDescriptions == null) return;
collectionView.GroupDescriptions.Add(new PropertyGroupDescription("Type"));

In the xaml

...

<Style x:Key="DataGridGroupHeaderCountStyle" TargetType="{x:Type GroupItem}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type GroupItem}">
                <Expander Background="{Binding Path=Tag,
                                               RelativeSource={RelativeSource Mode=FindAncestor,
                                                                              AncestorType=DataGrid}}"
                          BorderBrush="{Binding Path=Tag,
                                                RelativeSource={RelativeSource Mode=FindAncestor,
                                                                               AncestorType=DataGrid}}"
                          Foreground="{Binding Path=Tag,
                                               RelativeSource={RelativeSource Mode=FindAncestor,
                                                                              AncestorType=DataGrid}}"
                          Style="{StaticResource DataGridExpanderGroupStyle}">
                    <Expander.Header>
                        <DockPanel>
                            <TextBlock MinWidth="100"
                                       Margin="5,0,0,0"
                                       HorizontalAlignment="Left"
                                       VerticalAlignment="Center"
                                       Text="{Binding Path=Name}" />
                            <TextBlock HorizontalAlignment="Left"
                                       VerticalAlignment="Center"
                                       Text="{Binding Path=ItemCount}" />
                        </DockPanel>
                    </Expander.Header>
                    <Expander.Content>
                        <ItemsPresenter />
                    </Expander.Content>
                </Expander>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

...

<DataGrid x:Name="MyDataGrid"
                      Grid.Row="1"
                      Grid.Column="0"
                      Grid.ColumnSpan="2"
                      IsReadOnly="True"
                      AutoGenerateColumns="False"
                      CanUserAddRows="False"
                      CanUserDeleteRows="False"
                      CanUserSortColumns="True"
                      EnableRowVirtualization="True"
                      IsSynchronizedWithCurrentItem="True"
                      ItemsSource="{Binding Path=CollectionView,
                                            Mode=OneWay}"
                      ScrollViewer.IsDeferredScrollingEnabled="True"
                      SelectionChanged="MyDataGridOnSelectionChanged"
                      SelectionMode="Extended"
                      SelectorHelper.AutoScrollIntoView="True"
                      Tag="{Binding Path=ColorsTheme.Border}"

                      VirtualizingPanel.IsVirtualizingWhenGrouping="True">

                <DataGrid.GroupStyle>
                    <GroupStyle ContainerStyle="{StaticResource DataGridGroupHeaderCountStyle}">
                        <GroupStyle.Panel>
                            <ItemsPanelTemplate>
                                <DataGridRowsPresenter />
                            </ItemsPanelTemplate>
                        </GroupStyle.Panel>
                    </GroupStyle>
                </DataGrid.GroupStyle>
                <DataGrid.Columns>

Description of the Problem :

For instance, I have let's say 25 000 rows in the datagrid. The loading is OK with the virtualization.

My problem is when I click on the first (of the two) group to collapse it : I can never see the second group, it disappears ! Does Somebody know why ? or is this a bug ?

When I'm not using VirtualizingPanel.IsVirtualizingWhenGrouping="True", the loading is very (extremely) slow, but at least, the behaviour of the groups is normal, that means I can collapse the first group, I can see the second coming below.

Answer 1

Maybe a bit late, but could help. We just ran into this problem and figured out that the DataGridRowsPresenter in the groups that should appear kept their Visibility = Collapsed. As a result the group itself would have an ActualHeight of 0 and not render.

To solve this, we hacked(?) in this ItemPanel definition:

<DataGrid.ItemsPanel>
    <ItemsPanelTemplate>
        <DataGridRowsPresenter IsItemsHost="True" Visibility="{Binding RelativeSource={RelativeSource AncestorType={x:Type Expander}}, Path=IsExpanded, Converter={StaticResource BooleanToVisibilityConverter}}"/>
    </ItemsPanelTemplate>
</DataGrid.ItemsPanel>

Basically enforcing Visibility if the group was expanded. This seems to keep the benefits of the row virtualization when grouping and makes sure the groups that were outside the visible range pop into view.

Seeing that others had the same issue will probably go ahead and post this as a bug to Microsoft with this "hack" as a hint what might be overlooked in the virtualization, grouping and scroll viewer interaction.