Disable collapsibility of DataGrid group headers - wpf

Question

I have a UWP application with a DataGrid defined in xaml.
The DataGrid contains grouped data. When it displays, there is a downward-facing carat to the left of the group headings that can be clicked, causing that group to collapse. As far as I can see, this is a default behavior that we didn't specifically ask for.
My customer doesn't like this and wants me to remove the functionality. How can I do this?

Answer 1

You can disable the functionality by modifying the DataGridRowGroupHeader.HeaderStyle. Simple way is to add IsHitTestVisible to False. This will disable the expander so the group wont get Collapsed.
<controls:DataGrid>
<controls:DataGrid.RowGroupHeaderStyles>
<Style TargetType="controls:DataGridRowGroupHeader">
<Setter Property="IsHitTestVisible" Value="False"/>
</Style>
</controls:DataGrid.RowGroupHeaderStyles>
</controls:DataGrid>
Edit
Although it doesn't eliminate the indicator.
Here's new style with removing indicator
<controls:DataGrid>
<controls:DataGrid.RowGroupHeaderStyles>
<Style TargetType="controls:DataGridRowGroupHeader">
<Setter Property="IsTabStop" Value="False"/>
<Setter Property="IsHitTestVisible" Value="False"/>
<Setter Property="FontSize" Value="15"/>
<Setter Property="MinHeight" Value="32"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="controls:DataGridRowGroupHeader">
<Grid x:Name="RowGroupHeaderRoot" MinHeight="{TemplateBinding MinHeight}">
<Grid.Resources>
<ControlTemplate x:Key="ToggleButtonTemplate" TargetType="ToggleButton">
<Grid Background="{TemplateBinding Background}">
</Grid>
</ControlTemplate>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Rectangle x:Name="IndentSpacer" Grid.Column="1"/>
<ToggleButton x:Name="ExpanderButton" Grid.Column="2" Height="12" Width="12" Template="{StaticResource ToggleButtonTemplate}"
IsTabStop="False" Margin="12,0,0,0" Background="{TemplateBinding Background}" Foreground="{TemplateBinding Foreground}"/>
<StackPanel Grid.Column="3" Orientation="Horizontal" VerticalAlignment="Center" Margin="12,0,0,0">
<TextBlock x:Name="PropertyNameElement" Margin="4,0,0,0" Visibility="{TemplateBinding PropertyNameVisibility}" Style="{ThemeResource BodyTextBlockStyle}" Foreground="{TemplateBinding Foreground}"/>
<TextBlock x:Name="PropertyValueElement" Margin="4,0,0,0" Style="{ThemeResource BodyTextBlockStyle}" Foreground="{TemplateBinding Foreground}"/>
<TextBlock x:Name="ItemCountElement" Margin="4,0,0,0" Visibility="{TemplateBinding ItemCountVisibility}" Style="{ThemeResource BodyTextBlockStyle}" Foreground="{TemplateBinding Foreground}"/>
</StackPanel>
<Rectangle x:Name="CurrencyVisual" Grid.ColumnSpan="5"
StrokeThickness="1" Fill="Transparent"
HorizontalAlignment="Stretch" VerticalAlignment="Stretch" IsHitTestVisible="False" Opacity="0"/>
<Grid x:Name="FocusVisual" Grid.ColumnSpan="5" IsHitTestVisible="False" Opacity="0">
<Rectangle StrokeThickness="2" Fill="Transparent"
HorizontalAlignment="Stretch" VerticalAlignment="Stretch" IsHitTestVisible="False"/>
<Rectangle StrokeThickness="1" Fill="Transparent"
HorizontalAlignment="Stretch" VerticalAlignment="Stretch" IsHitTestVisible="False" Margin="2"/>
</Grid>
<Rectangle x:Name="BottomGridLine" Grid.ColumnSpan="5" Height="1" Grid.Row="1"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</controls:DataGrid.RowGroupHeaderStyles>
</controls:DataGrid>