Default combobox template. Popup width - wpf

Question

I want add some buttons to default combobox template, so I override its template.
I use this:
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="l:RefreshableComboBox">
<Grid Name="MainGrid" SnapsToDevicePixels="True">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="0" MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" />
</Grid.ColumnDefinitions>
<Popup IsOpen="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Bottom" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" AllowsTransparency="True" Name="PART_Popup" Margin="1,1,1,1" Grid.ColumnSpan="2" >
<mwt:SystemDropShadowChrome Color="#00FFFFFF" Name="Shdw" MinWidth="0" MaxHeight="{TemplateBinding ComboBox.MaxDropDownHeight}">
<Border BorderThickness="1,1,1,1" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" Name="DropDownBorder">
<ScrollViewer Name="DropDownScrollViewer">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas Width="0" Height="0" HorizontalAlignment="Left" VerticalAlignment="Top">
<Rectangle Fill="{x:Null}" Name="OpaqueRect" Width="Auto" Height="Auto" />
</Canvas>
<ItemsPresenter Name="ItemsPresenter" SnapsToDevicePixels="{TemplateBinding UIElement.SnapsToDevicePixels}" KeyboardNavigation.DirectionalNavigation="Contained" />
</Grid>
</ScrollViewer>
</Border>
</mwt:SystemDropShadowChrome>
</Popup>
<ToggleButton x:Name="ToggleButton" IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" BorderBrush="{TemplateBinding Border.BorderBrush}" Background="{TemplateBinding Panel.Background}" Grid.ColumnSpan="2">
<ToggleButton.Style>
<Style TargetType="ToggleButton">
<Style.Resources>
<ResourceDictionary />
</Style.Resources>
<Setter Property="FrameworkElement.OverridesDefaultStyle">
<Setter.Value>
<s:Boolean>True</s:Boolean>
</Setter.Value>
</Setter>
<Setter Property="KeyboardNavigation.IsTabStop">
<Setter.Value>
<s:Boolean>False</s:Boolean>
</Setter.Value>
</Setter>
<Setter Property="UIElement.Focusable">
<Setter.Value>
<s:Boolean>False</s:Boolean>
</Setter.Value>
</Setter>
<Setter Property="ButtonBase.ClickMode">
<Setter.Value>
<x:Static Member="ClickMode.Press" />
</Setter.Value>
</Setter>
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<mwt:ButtonChrome Background="{TemplateBinding Panel.Background}"
BorderBrush="{TemplateBinding Border.BorderBrush}"
RenderMouseOver="{TemplateBinding UIElement.IsMouseOver}"
RenderPressed="{TemplateBinding ButtonBase.IsPressed}"
Name="Chrome" SnapsToDevicePixels="True">
<Grid Width="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" HorizontalAlignment="Right">
<Path Data="M0,0L3.5,4 7,0z" Fill="#FF000000" Name="Arrow" Margin="3,1,0,0"
HorizontalAlignment="Center" VerticalAlignment="Center" />
</Grid>
</mwt:ButtonChrome>
<ControlTemplate.Triggers>
<Trigger Property="ToggleButton.IsChecked">
<Setter Property="mwt:ButtonChrome.RenderPressed" TargetName="Chrome">
<Setter.Value>
<s:Boolean>True</s:Boolean>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>True</s:Boolean>
</Trigger.Value>
</Trigger>
<Trigger Property="UIElement.IsEnabled">
<Setter Property="Shape.Fill" TargetName="Arrow">
<Setter.Value>
<SolidColorBrush>#FFAFAFAF</SolidColorBrush>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>False</s:Boolean>
</Trigger.Value>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ToggleButton.Style>
</ToggleButton>
<ContentPresenter Content="{TemplateBinding ComboBox.SelectionBoxItem}" ContentTemplate="{TemplateBinding ComboBox.SelectionBoxItemTemplate}" ContentStringFormat="{TemplateBinding ComboBox.SelectionBoxItemStringFormat}" Margin="{TemplateBinding Control.Padding}" HorizontalAlignment="{TemplateBinding Control.HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding Control.VerticalContentAlignment}" SnapsToDevicePixels="{TemplateBinding UIElement.SnapsToDevicePixels}" IsHitTestVisible="False" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Popup.HasDropShadow" SourceName="PART_Popup">
<Setter Property="FrameworkElement.Margin" TargetName="Shdw">
<Setter.Value>
<Thickness>0,0,5,5</Thickness>
</Setter.Value>
</Setter>
<Setter Property="mwt:SystemDropShadowChrome.Color" TargetName="Shdw">
<Setter.Value>
<Color>#71000000</Color>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>True</s:Boolean>
</Trigger.Value>
</Trigger>
<Trigger Property="ItemsControl.HasItems">
<Setter Property="FrameworkElement.Height" TargetName="DropDownBorder">
<Setter.Value>
<s:Double>95</s:Double>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>False</s:Boolean>
</Trigger.Value>
</Trigger>
<Trigger Property="UIElement.IsEnabled">
<Setter Property="TextElement.Foreground">
<Setter.Value>
<DynamicResource ResourceKey="{x:Static SystemColors.GrayTextBrushKey}" />
</Setter.Value>
</Setter>
<Setter Property="Panel.Background">
<Setter.Value>
<SolidColorBrush>#FFF4F4F4</SolidColorBrush>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>False</s:Boolean>
</Trigger.Value>
</Trigger>
<Trigger Property="ItemsControl.IsGrouping">
<Setter Property="ScrollViewer.CanContentScroll">
<Setter.Value>
<s:Boolean>False</s:Boolean>
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>True</s:Boolean>
</Trigger.Value>
</Trigger>
<Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer">
<Setter Property="Canvas.Top" TargetName="OpaqueRect">
<Setter.Value>
<Binding Path="VerticalOffset" ElementName="DropDownScrollViewer" />
</Setter.Value>
</Setter>
<Setter Property="Canvas.Left" TargetName="OpaqueRect">
<Setter.Value>
<Binding Path="HorizontalOffset" ElementName="DropDownScrollViewer" />
</Setter.Value>
</Setter>
<Trigger.Value>
<s:Boolean>False</s:Boolean>
</Trigger.Value>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
By some reason my popup(PART_Popup) isn't the same width as toggleButton. It has width enough for displaying item. If I don't override template popup is the same width as toggleButton. So I can't understand why I can't get the same situation with standart template

Answer 1

Change the MinWidth of mwt:SystemDropShadowChrome from 0 to:
MinWidth="{Binding ElementName=MainGrid,Path=ActualWidth}"