Semi.Avalonia/src/Semi.Avalonia/Controls/Expander.axaml

<ResourceDictionary
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:CompileBindings="True">
    <Design.PreviewWith>
        <StackPanel Margin="20" Spacing="20">
            <TextBox>Hello</TextBox>
            <Expander Header="Hello">Content</Expander>
        </StackPanel>
    </Design.PreviewWith>
    <ControlTheme x:Key="ExpanderHeaderToggleButtonTheme" TargetType="ToggleButton">
        <Setter Property="Template">
            <ControlTemplate TargetType="ToggleButton">
                <ContentPresenter
                    Padding="{TemplateBinding Padding}"
                    Background="{TemplateBinding Background}"
                    Content="{TemplateBinding Content}" />
            </ControlTemplate>
        </Setter>
    </ControlTheme>
    <ControlTheme x:Key="{x:Type Expander}" TargetType="Expander">
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="BorderBrush" Value="{DynamicResource ExpanderSeparatorBorderBrush}" />
        <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
        <Setter Property="HorizontalAlignment" Value="Stretch" />
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        <Setter Property="VerticalContentAlignment" Value="Stretch" />
        <Setter Property="Template">
            <ControlTemplate TargetType="Expander">
                <DockPanel>
                    <LayoutTransformControl Name="ExpanderHeaderLayoutContainer" Margin="{DynamicResource ExpanderHeaderMargin}">
                        <ToggleButton
                            Name="ExpanderHeader"
                            Padding="{DynamicResource ExpanderHeaderPadding}"
                            HorizontalAlignment="Stretch"
                            HorizontalContentAlignment="Stretch"
                            Background="{DynamicResource ExpanderHeaderDefaultBackground}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            IsChecked="{TemplateBinding IsExpanded, Mode=TwoWay}"
                            IsEnabled="{TemplateBinding IsEnabled}"
                            TextElement.FontWeight="{DynamicResource ExpanderHeaderFontWeight}"
                            TextElement.Foreground="{DynamicResource ExpanderHeaderForeground}"
                            Theme="{StaticResource ExpanderHeaderToggleButtonTheme}">
                            <Grid Background="Transparent" ColumnDefinitions="*, Auto">
                                <ContentPresenter
                                    Grid.Column="0"
                                    Content="{TemplateBinding Header}"
                                    ContentTemplate="{TemplateBinding HeaderTemplate}" />
                                <PathIcon
                                    Name="PART_PathIcon"
                                    Grid.Column="1"
                                    Theme="{DynamicResource InnerPathIcon}"
                                    Margin="{DynamicResource ExpanderIconMargin}"
                                    Data="{DynamicResource ExpanderIcon}">
                                    <PathIcon.Transitions>
                                        <Transitions>
                                            <TransformOperationsTransition Property="RenderTransform" Duration="0.1" />
                                        </Transitions>
                                    </PathIcon.Transitions>
                                </PathIcon>
                            </Grid>
                        </ToggleButton>
                    </LayoutTransformControl>
                    <LayoutTransformControl Name="ExpanderContentLayoutContainer" MinHeight="1">
                        <Border
                            Name="ExpanderContent"
                            MinHeight="2"
                            BorderBrush="{DynamicResource ExpanderSeparatorBorderBrush}"
                            BorderThickness="0,0,0,1">
                            <ContentPresenter
                                Name="PART_ContentPresenter"
                                Margin="{DynamicResource ExpanderContentMargin}"
                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                Content="{TemplateBinding Content}"
                                ContentTemplate="{TemplateBinding ContentTemplate}"
                                Foreground="{DynamicResource ExpanderContentForeground}"
                                IsVisible="{TemplateBinding IsExpanded, Mode=TwoWay}" />
                        </Border>
                    </LayoutTransformControl>
                </DockPanel>
            </ControlTemplate>
        </Setter>

        <Style Selector="^:expanded /template/ PathIcon#PART_PathIcon">
            <Setter Property="RenderTransform" Value="rotate(180deg)" />
        </Style>

        <Style Selector="^:right">
            <Style Selector="^ /template/ LayoutTransformControl#ExpanderHeaderLayoutContainer">
                <Setter Property="LayoutTransform">
                    <RotateTransform Angle="-90" />
                </Setter>
                <Setter Property="VerticalAlignment" Value="Stretch" />
            </Style>
            <Setter Property="HorizontalAlignment" Value="Left" />
            <Setter Property="VerticalAlignment" Value="Stretch" />
        </Style>

        <Style Selector="^:left">
            <Style Selector="^ /template/ LayoutTransformControl#ExpanderHeaderLayoutContainer">
                <Setter Property="LayoutTransform">
                    <RotateTransform Angle="90" />
                </Setter>
                <Setter Property="VerticalAlignment" Value="Stretch" />
            </Style>
            <Setter Property="HorizontalAlignment" Value="Stretch" />
            <Setter Property="VerticalAlignment" Value="Stretch" />
        </Style>

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

        <Style Selector="^:left /template/ Border#ExpanderContent">
            <Setter Property="MinWidth" Value="2" />
            <Setter Property="BorderThickness" Value="{DynamicResource ExpanderLeftContentBorderThickness}" />
        </Style>
        <Style Selector="^:up /template/ Border#ExpanderContent">
            <Setter Property="MinHeight" Value="2" />
            <Setter Property="BorderThickness" Value="{DynamicResource ExpanderUpContentBorderThickness}" />
        </Style>
        <Style Selector="^:right /template/ Border#ExpanderContent">
            <Setter Property="MinWidth" Value="2" />
            <Setter Property="BorderThickness" Value="{DynamicResource ExpanderRightContentBorderThickness}" />
        </Style>
        <Style Selector="^:down /template/ Border#ExpanderContent">
            <Setter Property="MinHeight" Value="2" />
            <Setter Property="BorderThickness" Value="{DynamicResource ExpanderDownContentBorderThickness}" />
        </Style>

        <Style Selector="^ /template/ ToggleButton#ExpanderHeader:pointerover">
            <Setter Property="Background" Value="{DynamicResource ExpanderHeaderHoverBackground}" />
        </Style>
        <Style Selector="^ /template/ ToggleButton#ExpanderHeader:pressed">
            <Setter Property="Background" Value="{DynamicResource ExpanderHeaderPressedBackground}" />
        </Style>
        <Style Selector="^ /template/ ToggleButton#ExpanderHeader:disabled">
            <Setter Property="Foreground" Value="{DynamicResource ExpanderHeaderDisabledForeground}" />
        </Style>
    </ControlTheme>
</ResourceDictionary>