Avalonia/samples/ControlCatalog/Pages/NavigationPage/NavigationPageModalPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.NavigationPageModalPage">
  <DockPanel>
    <ScrollViewer DockPanel.Dock="Right" Width="280">
      <StackPanel Margin="12" Spacing="16">
        <TextBlock Text="Configuration" FontSize="16" FontWeight="SemiBold"
                   Foreground="{DynamicResource SystemControlHighlightAccentBrush}" />

        <!-- Modal Actions -->
        <TextBlock Text="Modal Actions" FontSize="13" FontWeight="SemiBold" />
        <StackPanel Spacing="6">
          <Button x:Name="PushModalButton"
                  Content="Push Modal"
                  HorizontalAlignment="Stretch"
                  Click="OnPushModal" />
          <Button x:Name="PopModalButton"
                  Content="Pop Modal"
                  HorizontalAlignment="Stretch"
                  Click="OnPopModal" />
          <Button x:Name="PopAllModalsButton"
                  Content="Pop All Modals"
                  HorizontalAlignment="Stretch"
                  Click="OnPopAllModals" />
        </StackPanel>

        <Separator />

        <!-- Modal Transition -->
        <TextBlock Text="Modal Transition" FontSize="13" FontWeight="SemiBold" />
        <ComboBox x:Name="TransitionCombo"
                  SelectedIndex="0"
                  HorizontalAlignment="Stretch"
                  SelectionChanged="OnTransitionChanged">
          <ComboBoxItem Content="Slide from Bottom" />
          <ComboBoxItem Content="CrossFade" />
          <ComboBoxItem Content="None" />
        </ComboBox>

        <Separator />
        <TextBlock Text="Modal pages slide up from the bottom and dismiss by sliding back down. The ModalTransition property controls this animation independently from the stack PageTransition."
                   TextWrapping="Wrap" FontSize="12" Opacity="0.7" />

        <Separator />
        <TextBlock Text="Status" FontWeight="SemiBold" FontSize="13" />
        <TextBlock x:Name="StatusText"
                   Text="Modals: 0"
                   Opacity="0.7"
                   TextWrapping="Wrap" />
      </StackPanel>
    </ScrollViewer>

    <Border DockPanel.Dock="Right" Width="1" Background="{DynamicResource SystemControlForegroundBaseMediumLowBrush}" />

    <Border Margin="12"
            BorderBrush="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
            BorderThickness="1"
            CornerRadius="6"
            ClipToBounds="True">
      <NavigationPage x:Name="DemoNav" />
    </Border>
  </DockPanel>
</UserControl>