Avalonia/samples/ControlCatalog/Pages/ExpanderPage.xaml

<UserControl xmlns="https://github.com/avaloniaui">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">Expander</TextBlock>
    <TextBlock Classes="h2">Expands to show nested content</TextBlock>

    <StackPanel Orientation="Vertical"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Spacing="16">
      <Expander Header="Expand Up" ExpandDirection="Up">
        <StackPanel>
          <TextBlock>Expanded content</TextBlock>
        </StackPanel>
      </Expander>
      <Expander Header="Expand Down" ExpandDirection="Down">
        <StackPanel>
          <TextBlock>Expanded content</TextBlock>
        </StackPanel>
      </Expander>
      <Expander Header="Expand Left" ExpandDirection="Left">
        <StackPanel>
          <TextBlock>Expanded content</TextBlock>
        </StackPanel>
      </Expander>
      <Expander Header="Expand Right" ExpandDirection="Right">
        <StackPanel>
          <TextBlock>Expanded content</TextBlock>
        </StackPanel>
      </Expander>
    </StackPanel>    
  </StackPanel>
</UserControl>