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

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:converters="clr-namespace:Avalonia.Controls.Converters;assembly=Avalonia.Controls">

  <Design.PreviewWith>
    <Grid ColumnDefinitions="*,20,*"
          Height="300"
          Width="200"
          Margin="20">
      <ScrollViewer>
        <Border Height="1000" Background="#ccc"/>
      </ScrollViewer>
      <ScrollViewer Grid.Column="2" Classes="menuscroller">
        <Border Height="1000" Background="#ccc"/>
      </ScrollViewer>
    </Grid>
  </Design.PreviewWith>
  
  <Style Selector="ScrollViewer">
    <Setter Property="Background"
            Value="Transparent" />
    <Setter Property="Template">
      <ControlTemplate>
        <Grid ColumnDefinitions="*,Auto" RowDefinitions="*,Auto">
          <ScrollContentPresenter Name="PART_ContentPresenter"
                                  Grid.Row="0"
                                  Grid.Column="0"
                                  Grid.RowSpan="2"
                                  Grid.ColumnSpan="2"
                                  Background="{TemplateBinding Background}"
                                  CanHorizontallyScroll="{TemplateBinding CanHorizontallyScroll}"
                                  CanVerticallyScroll="{TemplateBinding CanVerticallyScroll}"
                                  Content="{TemplateBinding Content}"
                                  Extent="{TemplateBinding Extent, Mode=TwoWay}"
                                  Margin="{TemplateBinding Padding}"
                                  Offset="{TemplateBinding Offset, Mode=TwoWay}"
                                  Viewport="{TemplateBinding Viewport, Mode=TwoWay}">
            <ScrollContentPresenter.GestureRecognizers>
              <ScrollGestureRecognizer
                CanHorizontallyScroll="{TemplateBinding CanHorizontallyScroll}"
                CanVerticallyScroll="{TemplateBinding CanVerticallyScroll}" />
            </ScrollContentPresenter.GestureRecognizers>
          </ScrollContentPresenter>
          <ScrollBar Name="PART_HorizontalScrollBar"
                     AllowAutoHide="{TemplateBinding AllowAutoHide}"
                     Orientation="Horizontal"
                     LargeChange="{Binding LargeChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
                     SmallChange="{Binding SmallChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
                     Maximum="{TemplateBinding HorizontalScrollBarMaximum}"
                     Value="{TemplateBinding HorizontalScrollBarValue, Mode=TwoWay}"
                     ViewportSize="{TemplateBinding HorizontalScrollBarViewportSize}"
                     Visibility="{TemplateBinding HorizontalScrollBarVisibility}"
                     Grid.Row="1"
                     Focusable="False" />
          <ScrollBar Name="PART_VerticalScrollBar"
                     AllowAutoHide="{TemplateBinding AllowAutoHide}"
                     Orientation="Vertical"
                     LargeChange="{Binding LargeChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
                     SmallChange="{Binding SmallChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
                     Maximum="{TemplateBinding VerticalScrollBarMaximum}"
                     Value="{TemplateBinding VerticalScrollBarValue, Mode=TwoWay}"
                     ViewportSize="{TemplateBinding VerticalScrollBarViewportSize}"
                     Visibility="{TemplateBinding VerticalScrollBarVisibility}"
                     Grid.Column="1"
                     Focusable="False" />
          <Panel x:Name="PART_ScrollBarsSeparator" Grid.Row="1" Grid.Column="1" Background="{DynamicResource ScrollViewerScrollBarsSeparatorBackground}" />
        </Grid>
      </ControlTemplate>
    </Setter>
  </Style>

  <Style Selector="ScrollViewer /template/ Panel#PART_ScrollBarsSeparator">
    <Setter Property="Opacity" Value="0" />
    <Setter Property="Transitions">
      <Transitions>
        <DoubleTransition Property="Opacity" Duration="0:0:0.1" />
      </Transitions>
    </Setter>
  </Style>

  <Style Selector="ScrollViewer[IsExpanded=true] /template/ Panel#PART_ScrollBarsSeparator">
    <Setter Property="Opacity" Value="1" />
  </Style>

  <Style Selector="ScrollViewer.menuscroller">
    <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"
                    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"
                    Width="20"
                    Height="20" />
            </Viewbox>
          </RepeatButton>
          <ScrollContentPresenter Name="PART_ContentPresenter"
                                  CanHorizontallyScroll="{TemplateBinding CanHorizontallyScroll}"
                                  CanVerticallyScroll="{TemplateBinding CanVerticallyScroll}"
                                  Content="{TemplateBinding Content}"
                                  Extent="{TemplateBinding Extent, Mode=TwoWay}"
                                  Margin="{TemplateBinding Padding}"
                                  Offset="{TemplateBinding Offset, Mode=TwoWay}"
                                  Viewport="{TemplateBinding Viewport, Mode=TwoWay}"/>
        </DockPanel>
      </ControlTemplate>
    </Setter>
  </Style>
  <Style Selector="ScrollViewer.menuscroller /template/ RepeatButton > Viewbox > Path">
    <Setter Property="Fill" Value="{DynamicResource ScrollBarButtonArrowForeground}" />
  </Style>

  <Style Selector="ScrollViewer.menuscroller /template/ RepeatButton:pointerover > Viewbox > Path">
    <Setter Property="Fill" Value="{DynamicResource ScrollBarButtonArrowForegroundPointerOver}" />
  </Style>

</Styles>