Inexplicable Margins in TreeViewItem rendered with custom control template

Hi

I have a TreeView control that uses default System.Windows.Controls.TreeViewItems with their own control template (template XAML see below). The TreeListView exposes GridViewColumns just like a ListView ind GridViewMode. So it has a GridViewRowPresenter inside its template. This GridViewRowPresenter binds to a Columns collections and uses the CellTemplates provided by the 'client' to display the cell content.

Now there is one automatism that i can't explain or modify and that is that for each cell of each column the GridViewRowPresenter creates a ContentPresenter at runtime (which is normal). The ContentPresenter then presents the content of the cell based on the CellTemplate of the GridVilewColumn(s). This is fully OK.

The problem is that the ContentPresenter has its margin set to 6,0,6,0, so there is a gap of 6px on the left of each treeview item!

This is what I'd like to get rid of. I Want a margin of 0.

I cannot see where this margin is defined, Its not in the TreeViewItem template, not in the TreeListView template and also not in the (many) cell DataTemplates. It seems some automatism coming from the GridViewRowPresenter or ContentPresenter default styles, I suppose.

If I modify the Margin to 0 using Snoop, then I get the result i want.

Does anyone know, which style / template is responsible for these horizontal margins and how i could fix them?

Thanks a lot,
Chris

Btw plz have a look at the Control Template and also the
screen shots of Snoop and the App!

The control template is as follows:

<ControlTemplate x:Key="TreeListViewItemTemplate" TargetType="TreeViewItem"><!--  #region TreeViewItem Content  --><StackPanel HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Orientation="Vertical"                    ><StackPanel Orientation="Horizontal" x:Name="PART_HeaderPanel"><!--  #region "Normal" content  --><Border x:Name="Border"
                        Margin="0"
                        Background="Transparent"
                        BorderBrush="Transparent"
                        BorderThickness="1"
                        CornerRadius="2"><Border x:Name="InnerBorder"
                            BorderBrush="Transparent"
                            BorderThickness="0.5"
                            CornerRadius="1.5"><!--  #region GridViewRowPresenter containing the Columns and their CellTemplates  --><GridViewRowPresenter x:Name="PART_Header"
                                              Grid.Column="1"
                                              Columns="{Binding Path=Columns, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=primitives:TreeListView}}"
                                              Content="{TemplateBinding Header}"<!-- ContentPresenters with Margins generated here , one for each column! -->
                                              Focusable="True" /><!--  #endregion  --></Border></Border><!--  #endregion  --></StackPanel><!--  #region ItemsPresenter containing ItemsPanel to display the sub nodes  --><ItemsPresenter x:Name="ItemsPresenter"
                            Focusable="True"
                            Visibility="Collapsed" /><!--  #endregion  --></StackPanel><ControlTemplate.Triggers><!--  lots of triggers for selection / focus / etc   --></ControlTemplate.Triggers>

Image Snoop:

Image Margin in app at runtime: