Indeed, what you want to do is edit the HierarchicalDataTemplate and change the way it behaves. The next page has a pretty good overview of the high level editing Hierarchical data template .
I also found this one to be very good to start with. Although none of the pages says what to do, you significantly change the layout properties in the element presenter.
Edit
Oops, I was wrong. Not HierarchicalDataTemplate, but TreeViewItem template.
See below for an example. This is simply the easiest way to do this if you KNOW that there will be no third level nodes.
Pay particular attention to the ItemsPresenter element named ItemsHost. He has a stock of -12,0,0,0. This means that its left margin is negative and therefore leaves the column of the grid containing it in the left direction. Thus, all child nodes are pulled a little to the left. If in the future you have nodes of the third level, they will also be pulled to the left. If you do not want this, then you will have to provide different templates for different levels of nodes. But this is beyond the scope of this answer.
<Style x:Key="TreeViewItemStyle1" TargetType="{x:Type TreeViewItem}"> <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="{x:Type TreeViewItem}"> <Grid> <Grid.ColumnDefinitions> <ColumnDefinition MinWidth="19" Width="Auto"/> <ColumnDefinition Width="Auto"/> <ColumnDefinition Width="*"/> </Grid.ColumnDefinitions> <Grid.RowDefinitions> <RowDefinition Height="Auto"/> <RowDefinition/> </Grid.RowDefinitions> <ToggleButton x:Name="Expander" ClickMode="Press" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ExpandCollapseToggleStyle}"/> <Border x:Name="Bd" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="1" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="true"> <ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/> </Border> <ItemsPresenter x:Name="ItemsHost" Grid.ColumnSpan="2" Grid.Column="1" Grid.Row="1" Margin="-12,0,0,0"/> </Grid> <ControlTemplate.Triggers> <Trigger Property="IsExpanded" Value="false"> <Setter Property="Visibility" TargetName="ItemsHost" Value="Collapsed"/> </Trigger> <Trigger Property="HasItems" Value="false"> <Setter Property="Visibility" TargetName="Expander" Value="Hidden"/> </Trigger> <Trigger Property="IsSelected" Value="true"> <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/> </Trigger> <MultiTrigger> <MultiTrigger.Conditions> <Condition Property="IsSelected" Value="true"/> <Condition Property="IsSelectionActive" Value="false"/> </MultiTrigger.Conditions> <Setter Property="Background" TargetName="Bd" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/> </MultiTrigger> <Trigger Property="IsEnabled" Value="false"> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/> </Trigger> </ControlTemplate.Triggers> </ControlTemplate> </Setter.Value> </Setter> <Style.Triggers> <Trigger Property="VirtualizingStackPanel.IsVirtualizing" Value="true"> <Setter Property="ItemsPanel"> <Setter.Value> <ItemsPanelTemplate> <VirtualizingStackPanel/> </ItemsPanelTemplate> </Setter.Value> </Setter> </Trigger> </Style.Triggers> </Style>
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