Avalonia/samples/ControlCatalog/Pages/CheckBoxPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.CheckBoxPage" Background="White">
  <Grid>
    <TextBlock x:Name="myTb" />
    <ToggleButton x:Name="TogglePopupButton" Height="30" Width="150" HorizontalAlignment="Left">
      <StackPanel>
        <TextBlock VerticalAlignment="Center" HorizontalAlignment="Center">
          <Run Text="Is button toggled? " />
          <Run Text="{Binding IsChecked, ElementName=TogglePopupButton}" />
        </TextBlock>

        <Popup Name="myPopup" IsOpen="{Binding IsChecked, ElementName=TogglePopupButton}">
          <Panel>
            <Border BorderThickness="1" IsHitTestVisible="False" Margin="0" BorderBrush="Red" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" />

            <TextBlock Name="myPopupText" Background="LightBlue" Foreground="Blue" Padding="30" Margin="150">
              Popup Text
            </TextBlock>
          </Panel>
        </Popup>
      </StackPanel>
    </ToggleButton>
  </Grid>
</UserControl>