Avalonia/src/Avalonia.Themes.Fluent/Controls/MenuScrollViewer.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:converters="using:Avalonia.Controls.Converters"
                    x:ClassModifier="internal">

  <Design.PreviewWith>
    <ScrollViewer Theme="{DynamicResource FluentMenuScrollViewer}" Width="200" Height="200">
      <StackPanel Spacing="20">
        <TextBlock>Item 1</TextBlock>
        <TextBlock>Item 2</TextBlock>
        <TextBlock>Item 3</TextBlock>
        <TextBlock>Item 4</TextBlock>
        <TextBlock>Item 5</TextBlock>
        <TextBlock>Item 6</TextBlock>
        <TextBlock>Item 7</TextBlock>
        <TextBlock>Item 8</TextBlock>
        <TextBlock>Item 9</TextBlock>
      </StackPanel>
    </ScrollViewer>
  </Design.PreviewWith>

  <ControlTheme x:Key="FluentMenuScrollViewer" TargetType="ScrollViewer">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="IsScrollChainingEnabled" Value="False" />
    <Setter Property="Template">
      <ControlTemplate>
        <DockPanel>
          <RepeatButton DockPanel.Dock="Top"
                        BorderThickness="0"
                        Background="Transparent"
                        HorizontalAlignment="Stretch"
                        HorizontalContentAlignment="Center"
                        RenderTransform="{x:Null}"
                        Command="{Binding LineUp, RelativeSource={RelativeSource TemplatedParent}}">
            <RepeatButton.IsVisible>
              <MultiBinding Converter="{x:Static converters:MenuScrollingVisibilityConverter.Instance}"
                            ConverterParameter="0">
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="VerticalScrollBarVisibility" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Offset.Y" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Extent.Height" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Viewport.Height" />
              </MultiBinding>
            </RepeatButton.IsVisible>
            <Viewbox Width="{DynamicResource ScrollBarButtonArrowIconFontSize}"
                     Height="{DynamicResource ScrollBarButtonArrowIconFontSize}">
              <Path VerticalAlignment="Center"
                    HorizontalAlignment="Center"
                    Data="M 19.091797 14.970703 L 10 5.888672 L 0.908203 14.970703 L 0.029297 14.091797 L 10 4.111328 L 19.970703 14.091797 Z"
                    Fill="{DynamicResource ScrollBarButtonArrowForeground}"
                    Width="20"
                    Height="20" />
            </Viewbox>
          </RepeatButton>
          <RepeatButton DockPanel.Dock="Bottom"
                        BorderThickness="0"
                        Background="Transparent"
                        HorizontalAlignment="Stretch"
                        HorizontalContentAlignment="Center"
                        RenderTransform="{x:Null}"
                        Command="{Binding LineDown, RelativeSource={RelativeSource TemplatedParent}}">
            <RepeatButton.IsVisible>
              <MultiBinding Converter="{x:Static converters:MenuScrollingVisibilityConverter.Instance}"
                            ConverterParameter="100">
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="VerticalScrollBarVisibility" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Offset.Y" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Extent.Height" />
                <Binding RelativeSource="{RelativeSource TemplatedParent}" Path="Viewport.Height" />
              </MultiBinding>
            </RepeatButton.IsVisible>
            <Viewbox Width="{DynamicResource ScrollBarButtonArrowIconFontSize}"
                     Height="{DynamicResource ScrollBarButtonArrowIconFontSize}">
              <Path VerticalAlignment="Center"
                    HorizontalAlignment="Center"
                    Data="M 18.935547 4.560547 L 19.814453 5.439453 L 10 15.253906 L 0.185547 5.439453 L 1.064453 4.560547 L 10 13.496094 Z"
                    Fill="{DynamicResource ScrollBarButtonArrowForeground}"
                    Width="20"
                    Height="20" />
            </Viewbox>
          </RepeatButton>
          <ScrollContentPresenter Name="PART_ContentPresenter">
            <ScrollContentPresenter.GestureRecognizers>
              <ScrollGestureRecognizer CanHorizontallyScroll="{Binding CanHorizontallyScroll, ElementName=PART_ContentPresenter}"
                                       CanVerticallyScroll="{Binding CanVerticallyScroll, ElementName=PART_ContentPresenter}" />
            </ScrollContentPresenter.GestureRecognizers>
          </ScrollContentPresenter>
        </DockPanel>
      </ControlTemplate>
    </Setter>
    <Style Selector="^ /template/ RepeatButton:pointerover > Viewbox > Path">
      <Setter Property="Fill" Value="{DynamicResource ScrollBarButtonArrowForegroundPointerOver}" />
    </Style>
  </ControlTheme>
</ResourceDictionary>