WPF 隐藏式控件

<Grid>
        <DockPanel>
            <StackPanel  Background="Red" Height="40" DockPanel.Dock="Top">
                <Button x:Name="ShowPanelBtn"  Height="40"  Width="40"    HorizontalAlignment="Right">
                    <Button.Style>
                        <Style TargetType="Button">
                            <Style.Triggers>
                                <Trigger Property="IsMouseOver" Value="true">
                                    <Setter Property="Content" Value="∨"/>
                                </Trigger>
                                <Trigger Property="IsMouseOver" Value="false">
                                    <Setter Property="Content" Value="∧"/>
                                </Trigger>
                            </Style.Triggers>
                        </Style>
                    </Button.Style>
                </Button>
            </StackPanel>
            <Grid Background="Green" x:Name="DownPanel"        Height="40"  VerticalAlignment="Top">
                <Grid.Style>
                    <Style TargetType="Grid">
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding ElementName=ShowPanelBtn,Path=IsMouseOver}" Value="true">
                                <Setter Property="Visibility" Value="Visible"/>
                            </DataTrigger>
                            <DataTrigger Binding="{Binding ElementName=ShowPanelBtn,Path=IsMouseOver}" Value="false">
                                <Setter Property="Visibility" Value="Collapsed"/>
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </Grid.Style>
            </Grid>
        </DockPanel>
    </Grid>