Fix: 菜单上下两个滚动按钮同时出现的问题

src/Shared/HandyControl_Shared/Themes/Styles/ScrollViewer.xaml

@@ -15,7 +15,7 @@
 
     <Style BasedOn="{StaticResource ScrollViewerBaseStyle}" TargetType="hc:ScrollViewer"/>
 
-    <Style x:Key="ScrollViewerUpDownRepeatButtonStyle" BasedOn="{StaticResource RepeatButtonIcon}" TargetType="RepeatButton">
+    <Style x:Key="ScrollViewerUpRepeatButtonStyle" BasedOn="{StaticResource RepeatButtonIcon}" TargetType="RepeatButton">
         <Setter Property="HorizontalAlignment" Value="Stretch"/>
         <Setter Property="Height" Value="10"/>
         <Setter Property="Padding" Value="0,2"/>
@@ -31,6 +31,23 @@
             </Setter.Value>
         </Setter>
     </Style>
+
+    <Style x:Key="ScrollViewerDownRepeatButtonStyle" BasedOn="{StaticResource RepeatButtonIcon}" TargetType="RepeatButton">
+        <Setter Property="HorizontalAlignment" Value="Stretch"/>
+        <Setter Property="Height" Value="10"/>
+        <Setter Property="Padding" Value="0,2"/>
+        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}"/>
+        <Setter Property="Visibility">
+            <Setter.Value>
+                <MultiBinding FallbackValue="Visibility.Collapsed" Converter="{StaticResource MenuScrollingVisibilityConverter}" ConverterParameter="100">
+                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="ComputedVerticalScrollBarVisibility" />
+                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="VerticalOffset" />
+                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="ExtentHeight" />
+                    <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="ViewportHeight" />
+                </MultiBinding>
+            </Setter.Value>
+        </Setter>
+    </Style>
     
     <ControlTemplate x:Key="ScrollViewerUpDownControlTemplate" TargetType="ScrollViewer">
         <Grid x:Name="Grid" Background="{TemplateBinding Background}">
@@ -39,9 +56,9 @@
                 <RowDefinition/>
                 <RowDefinition Height="Auto"/>
             </Grid.RowDefinitions>
-            <RepeatButton Style="{StaticResource ScrollViewerUpDownRepeatButtonStyle}" hc:IconElement.Geometry="{StaticResource UpGeometry}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="{x:Static ScrollBar.LineUpCommand}"/>
+            <RepeatButton Style="{StaticResource ScrollViewerUpRepeatButtonStyle}" hc:IconElement.Geometry="{StaticResource UpGeometry}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="{x:Static ScrollBar.LineUpCommand}"/>
             <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" CanHorizontallyScroll="False" CanVerticallyScroll="False" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="0" Margin="{TemplateBinding Padding}" Grid.Row="1"/>
-            <RepeatButton Style="{StaticResource ScrollViewerUpDownRepeatButtonStyle}" hc:IconElement.Geometry="{StaticResource DownGeometry}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Row="2" Command="{x:Static ScrollBar.LineDownCommand}"/>
+            <RepeatButton Style="{StaticResource ScrollViewerDownRepeatButtonStyle}" hc:IconElement.Geometry="{StaticResource DownGeometry}" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Grid.Row="2" Command="{x:Static ScrollBar.LineDownCommand}"/>
         </Grid>
     </ControlTemplate>