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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:CompileBindings="True">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Orientation="Vertical" Spacing="20" Width="350" Height="600">
        <Expander ExpandDirection="Up" Header="Expand Up">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Down" Header="Expand Down">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Left" Header="Expand Left">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Right" Header="Expand Right">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Up">
          <Expander.Header>
            <Grid ColumnDefinitions="*, Auto">
              <TextBlock Grid.Column="0" Text="Control" />
              <TextBlock Grid.Column="1" Text="Header" />
            </Grid>
          </Expander.Header>
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
        <Expander ExpandDirection="Up" Header="Rounded" CornerRadius="25">
          <StackPanel>
            <TextBlock>Expanded content</TextBlock>
          </StackPanel>
        </Expander>
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <Thickness x:Key="ExpanderHeaderPadding">16</Thickness>
  <Thickness x:Key="ExpanderContentPadding">16</Thickness>
  <Thickness x:Key="ExpanderBorderThickness">1</Thickness>
  <Thickness x:Key="ExpanderDropdownLeftBorderThickness">1,1,0,1</Thickness>
  <Thickness x:Key="ExpanderDropdownUpBorderThickness">1,1,1,0</Thickness>
  <Thickness x:Key="ExpanderDropdownRightBorderThickness">0,1,1,1</Thickness>
  <Thickness x:Key="ExpanderDropdownDownBorderThickness">1,0,1,1</Thickness>
  <SolidColorBrush x:Key="ExpanderBackground" Color="{DynamicResource SystemAltMediumHighColor}" />
  <SolidColorBrush x:Key="ExpanderBorderBrush" Color="{DynamicResource SystemBaseLowColor}" />
  <SolidColorBrush x:Key="ExpanderDropDownBackground" Color="{DynamicResource SystemChromeMediumLowColor}" />
  <SolidColorBrush x:Key="ExpanderDropDownBorderBrush" Color="{DynamicResource SystemBaseLowColor}" />
  <SolidColorBrush x:Key="ExpanderForeground" Color="{DynamicResource SystemBaseHighColor}" />
  <SolidColorBrush x:Key="ExpanderChevronForeground" Color="{DynamicResource SystemBaseHighColor}" />

  <ControlTheme x:Key="FluentExpanderToggleButtonTheme" TargetType="ToggleButton">
    <Setter Property="Template">
      <ControlTemplate>
        <Border x:Name="ToggleButtonBackground"
                Background="{TemplateBinding Background}"
                BorderBrush="{TemplateBinding BorderBrush}"
                CornerRadius="{TemplateBinding CornerRadius}"
                BorderThickness="{TemplateBinding BorderThickness}">
          <Grid ColumnDefinitions="*,Auto">
            <ContentPresenter x:Name="PART_ContentPresenter"
                              Margin="{TemplateBinding Padding}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="Center"
                              Background="Transparent"
                              BorderBrush="Transparent"
                              BorderThickness="0"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Foreground="{DynamicResource ExpanderForeground}" />
            <Border x:Name="ExpandCollapseChevronBorder"
                    Grid.Column="1"
                    Width="32"
                    Height="32"
                    Margin="7"
                    RenderTransformOrigin="50%,50%">
              <Path x:Name="ExpandCollapseChevron"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center"
                    RenderTransformOrigin="50%,50%"
                    Stretch="None"
                    Stroke="{DynamicResource ExpanderChevronForeground}"
                    StrokeThickness="1" />
              <Border.RenderTransform>
                <RotateTransform />
              </Border.RenderTransform>
            </Border>
          </Grid>
        </Border>
      </ControlTemplate>
    </Setter>
    <Style Selector="^:checked /template/ Border#ExpandCollapseChevronBorder">
      <Style.Animations>
        <Animation FillMode="Both" Duration="0:0:0.0625">
          <KeyFrame Cue="100%">
            <Setter Property="RotateTransform.Angle" Value="180" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>
    <Style Selector="^:not(:checked) /template/ Border#ExpandCollapseChevronBorder">
      <Style.Animations>
        <Animation FillMode="Both" Duration="0:0:0.0625">
          <KeyFrame Cue="0%">
            <Setter Property="RotateTransform.Angle" Value="180" />
          </KeyFrame>
          <KeyFrame Cue="100%">
            <Setter Property="RotateTransform.Angle" Value="0" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
    </Style>
  </ControlTheme>
  <ControlTheme x:Key="FluentExpanderToggleButtonUpTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 7 L 7 0 L 14 7" />
    </Style>
  </ControlTheme>
  <ControlTheme x:Key="FluentExpanderToggleButtonDownTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 0 L 7 7 L 14 0" />
    </Style>
  </ControlTheme>
  <ControlTheme x:Key="FluentExpanderToggleButtonLeftTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 7 0 L 0 7 L 7 14" />
    </Style>
  </ControlTheme>
  <ControlTheme x:Key="FluentExpanderToggleButtonRightTheme" TargetType="ToggleButton" BasedOn="{StaticResource FluentExpanderToggleButtonTheme}">
    <Style Selector="^ /template/ Path#ExpandCollapseChevron">
      <Setter Property="Data" Value="M 0 0 L 7 7 L 0 14" />
    </Style>
  </ControlTheme>
  <ControlTheme x:Key="{x:Type Expander}" TargetType="Expander">
    <Setter Property="Background" Value="{DynamicResource ExpanderBackground}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ExpanderBorderThickness}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ExpanderBorderBrush}" />
    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
    <Setter Property="Padding" Value="{DynamicResource ExpanderHeaderPadding}" />
    <Setter Property="HorizontalAlignment" Value="Stretch" />
    <Setter Property="HorizontalContentAlignment" Value="Left" />
    <Setter Property="VerticalContentAlignment" Value="Center" />
    <Setter Property="Template">
      <ControlTemplate>
        <DockPanel>
          <ToggleButton x:Name="ExpanderHeader"
                        Padding="{TemplateBinding Padding}"
                        HorizontalAlignment="{TemplateBinding HorizontalAlignment}"
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        HorizontalContentAlignment="Stretch"
                        Content="{TemplateBinding Header}"
                        ContentTemplate="{TemplateBinding HeaderTemplate}"
                        IsChecked="{TemplateBinding IsExpanded, Mode=TwoWay}"
                        IsEnabled="{TemplateBinding IsEnabled}"/>
          <Border x:Name="ExpanderContent"
                  Padding="{DynamicResource ExpanderContentPadding}"
                  Background="{DynamicResource ExpanderDropDownBackground}"
                  BorderBrush="{DynamicResource ExpanderDropDownBorderBrush}"
                  IsVisible="{TemplateBinding IsExpanded, Mode=TwoWay}">
            <ContentPresenter x:Name="PART_ContentPresenter"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                              Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}" />
          </Border>
        </DockPanel>
      </ControlTemplate>
    </Setter>

    <Style Selector="^ /template/ ToggleButton#ExpanderHeader:pressed">
      <Setter Property="RenderTransform" Value="{x:Null}" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader, ^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="VerticalAlignment" Value="Stretch" />
    </Style>

    <Style Selector="^:not(:expanded) /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{Binding $parent[Expander].CornerRadius}" />
    </Style>
    <Style Selector="^:expanded:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource BottomCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:up /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource TopCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource TopCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:down /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource BottomCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource RightCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:left /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource LeftCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource LeftCornerRadiusFilterConverter}}" />
    </Style>
    <Style Selector="^:expanded:right /template/ Border#ExpanderContent">
      <Setter Property="CornerRadius" Value="{TemplateBinding CornerRadius, Converter={StaticResource RightCornerRadiusFilterConverter}}" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Right" />
    </Style>
    <Style Selector="^:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Bottom" />
    </Style>
    <Style Selector="^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Left" />
    </Style>
    <Style Selector="^:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="DockPanel.Dock" Value="Top" />
    </Style>

    <Style Selector="^:left /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{StaticResource FluentExpanderToggleButtonLeftTheme}" />
    </Style>
    <Style Selector="^:up /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{StaticResource FluentExpanderToggleButtonUpTheme}" />
    </Style>
    <Style Selector="^:right /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{StaticResource FluentExpanderToggleButtonRightTheme}" />
    </Style>
    <Style Selector="^:down /template/ ToggleButton#ExpanderHeader">
      <Setter Property="Theme" Value="{StaticResource FluentExpanderToggleButtonDownTheme}" />
    </Style>

    <Style Selector="^:left /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderDropdownLeftBorderThickness}" />
    </Style>
    <Style Selector="^:up /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderDropdownUpBorderThickness}" />
    </Style>
    <Style Selector="^:right /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderDropdownRightBorderThickness}" />
    </Style>
    <Style Selector="^:down /template/ Border#ExpanderContent">
      <Setter Property="BorderThickness" Value="{DynamicResource ExpanderDropdownDownBorderThickness}" />
    </Style>
  </ControlTheme>
</ResourceDictionary>