当ScrollBar可见时,WPF ScrollViewer显示边框

   <Style TargetType="{x:Type ScrollViewer}">
       <Setter Property="Template">
           <Setter.Value>
               <ControlTemplate TargetType="{x:Type ScrollViewer}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                           <ColumnDefinition Width="*"/>
                           <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="*"/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>
                        <Rectangle Grid.Column="1" Grid.Row="1" Fill="Black" Width="1" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"/>
                        <Rectangle Grid.Column="0" Grid.Row="1" Fill="Black" Height="1" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"/>
                       <ScrollContentPresenter Grid.Column="0" Grid.Row="0" Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
                       <ScrollBar Grid.Column="1" Grid.Row="0" Name="PART_VerticalScrollBar" Orientation="Vertical" Maximum="{TemplateBinding ScrollableHeight}" ViewportSize="{TemplateBinding ViewportHeight}" Value="{TemplateBinding VerticalOffset}" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"/>
                       <ScrollBar Grid.Column="0" Grid.Row="1" Name="PART_HorizontalScrollBar" Orientation="Horizontal" Maximum="{TemplateBinding ScrollableWidth}" ViewportSize="{TemplateBinding ViewportWidth}" Value="{TemplateBinding HorizontalOffset}" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>