WPF sharedsizegroup

Question

I have a problem with using a shared size group.

It works fine until I change the textbox value:

After changing the textbox's value, which will reduce the textbox size:

I think the problem is in the update, because when I try to select a grid through a "live visual tree" problem space disappear.

Can someone know how to fix this problem ?

Control template

<ControlTemplate TargetType="{x:Type Controls:ElementEditor}" >
                <Border Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}" x:Name="MainBorder">

                    <StackPanel Orientation="Horizontal" x:Name="MainStack" DataContext="{Binding Path=.,  RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}">
                        <Border Visibility="{Binding Element.Value, Converter={StaticResource NullToVisRev}, ConverterParameter=Collapsed}" HorizontalAlignment="Left">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition>
                                        <ColumnDefinition.SharedSizeGroup>
                                            <MultiBinding Converter="{StaticResource ColumnGroupNameGen}">
                                                <Binding Path="Element.Name" UpdateSourceTrigger="PropertyChanged"/>
                                                <Binding Path="ParentElement.Name" UpdateSourceTrigger="PropertyChanged"/>
                                            </MultiBinding>
                                        </ColumnDefinition.SharedSizeGroup>
                                    </ColumnDefinition>
                                </Grid.ColumnDefinitions>

                                <TextBox x:Name="PART_cellText" Text="{Binding Path=Element.Name,UpdateSourceTrigger=PropertyChanged}" Style="{DynamicResource HeaderTextbox}" />
                            </Grid>
                        </Border>
                        <StackPanel>
                            <TextBox Text="{Binding Path=Element.Name,UpdateSourceTrigger=PropertyChanged}" 
                                         HorizontalAlignment="Stretch" 
                                         VerticalAlignment="Stretch" 
                                         x:Name="Header" 
                                         Visibility="{Binding Element.Value, Converter={StaticResource NullToVis}, ConverterParameter=Collapsed}"
                                         Style="{DynamicResource HeaderTextbox}"/>
                            <ItemsControl ItemsSource="{Binding Element.Value}">
                                <ItemsControl.ItemTemplate>
                                    <DataTemplate>
                                        <Controls:ElementEditor x:Name="PART_Header"
                                            Element="{Binding}" 
                                            ParentElement="{Binding ElementName=MainStack, Path=DataContext.Element,Mode=OneWay,UpdateSourceTrigger=PropertyChanged}" 
                                            Style="{DynamicResource ElementHeaderTable}"/>
                                    </DataTemplate>

                                </ItemsControl.ItemTemplate>

                                <ItemsControl.ItemsPanel>
                                    <ItemsPanelTemplate>
                                        <StackPanel Orientation="Horizontal"/>
                                    </ItemsPanelTemplate>
                                </ItemsControl.ItemsPanel>
                            </ItemsControl>
                        </StackPanel>
                    </StackPanel>
                </Border>
            </ControlTemplate>

Group name Converter

public object Convert(object[] value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value[0]!=null)
        {
            string GroupName;

            GroupName = value[0].ToString();

            if (value[1] != null)
            {
                GroupName = value[1].ToString() + GroupName;
            }

            System.Diagnostics.Debug.WriteLine(GroupName);
            return String.Format("Group{0}", GroupName);
        }

        return 0;
    }