//Если SL 4-й версии
            if (Environment.Version.ToString()[0] == '4')
            {
                var sourceOffset = this.TransformToVisual(Application.Current.RootVisual).Transform(new Point(0, 0));
                popup.HorizontalOffset = sourceOffset.X + this.RenderSize.Width;
                popup.VerticalOffset = sourceOffset.Y;
            }

<Style TargetType="local:SuperMenuItem">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="BorderBrush" Value="Transparent"/>
        <Setter Property="Padding" Value="4,3,2,3"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:SuperMenuItem">
                    <Grid x:Name="PART_RootGrid">

                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="Normal"/>
                                <VisualState x:Name="Disabled">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0" To="0.5" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="Presenter"/>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                            <VisualStateGroup x:Name="FocusStates">
                                <VisualState x:Name="Unfocused"/>
                                <VisualState x:Name="Focused">
                                    <Storyboard>
                                        <DoubleAnimation Duration="0" To="1" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="Bg"/>
                                        <ColorAnimation Duration="0" To="#40FFFFFF" Storyboard.TargetProperty="(Shape.Stroke).(SolidColorBrush.Color)" Storyboard.TargetName="InnerBorder"/>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Rectangle Fill="{TemplateBinding Background}" RadiusY="2" RadiusX="2" Stroke="{TemplateBinding BorderBrush}" StrokeThickness="1"/>
                        <Rectangle x:Name="Bg" Opacity="0" RadiusY="2" RadiusX="2" Stroke="#8071CBF1" StrokeThickness="1">
                            <Rectangle.Fill>
                                <LinearGradientBrush EndPoint="0,1" StartPoint="0,0">
                                    <GradientStop Color="#34C5EBFF" Offset="0"/>
                                    <GradientStop Color="#3481D8FF" Offset="1"/>
                                </LinearGradientBrush>
                            </Rectangle.Fill>
                        </Rectangle>
                        <Rectangle x:Name="InnerBorder" Margin="1" RadiusY="2" RadiusX="2" Stroke="Transparent"/>

                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition MinWidth="24" Width="Auto" />
                                <ColumnDefinition Width="4" />
                                <ColumnDefinition Width="*" />
                                <ColumnDefinition Width="17" />
                            </Grid.ColumnDefinitions>

                            <ContentPresenter Content="{TemplateBinding Icon}" Margin="1" VerticalAlignment="Center"/>
                            <ContentPresenter x:Name="Presenter" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.Column="2" Margin="{TemplateBinding Padding}" />
                            <Path x:Name="PART_RightIcon" Grid.Column="3" Data="M 0,0 L 4,3.5 L 0,7 Z" Fill="Black" Margin="4,0,0,0" VerticalAlignment="Center" Visibility="{TemplateBinding HasSubItems}" />
                            <!--<Path x:Name="Glyph" Data="M 0,5.1 L 1.7,5.2 L 3.4,7.1 L 8,0.4 L 9.2,0 L 3.3,10.8 Z"
                                  Fill="#0C12A1" FlowDirection="LeftToRight" Height="11" Width="9"/>-->
                        </Grid>

                        <Popup x:Name="PART_Popup" HorizontalOffset="{TemplateBinding ActualWidth}" IsOpen="{TemplateBinding IsSubmenuOpen}" Margin="-4,0,0,0">
                            <ContentControl x:Name="SubMenuBorder">
                                <ContentControl.Template>
                                    <ControlTemplate>
                                        <Grid Background="#FFF5F5F5">
                                            <Rectangle Fill="#F1F1F1" HorizontalAlignment="Left" RadiusY="2" RadiusX="2" Width="28" />
                                            <Rectangle Fill="#E2E3E3" HorizontalAlignment="Left" Width="1" Margin="30,0,0,0" />
                                            <Rectangle Fill="White" HorizontalAlignment="Left" Width="1" Margin="31,0,0,0" />
                                            <ContentPresenter Grid.ColumnSpan="2" Margin="1,0" />
                                        </Grid>
                                    </ControlTemplate>
                                </ContentControl.Template>
                                <ScrollViewer x:Name="SubMenuScrollViewer" VerticalScrollBarVisibility="Auto" Padding="0">
                                    <Grid>
                                        <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                                            <Rectangle Fill="#FFF5F5F5" Height="{Binding ActualHeight, ElementName=SubMenuBorder}" Width="{Binding ActualWidth, ElementName=SubMenuBorder}" />
                                        </Canvas>
                                        <ItemsPresenter x:Name="ItemsPresenter" Margin="2" />
                                    </Grid>
                                </ScrollViewer>
                            </ContentControl>
                        </Popup>

                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>