Avalonia/samples/ControlCatalog/Pages/ChildClippingPage.xaml

<UserControl x:Class="ControlCatalog.Pages.ChildClippingPage"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:pages="using:ControlCatalog.Pages"
             mc:Ignorable="d"
             d:DesignHeight="800"
             d:DesignWidth="400">
  <ScrollViewer>
    <StackPanel Margin="16" Spacing="16">
      <TextBlock Classes="h2">Child Clipping</TextBlock>
      <TextBlock TextWrapping="Wrap">
        Border and ContentPresenter now clip only their children when ClipToBounds is enabled.
        The control's own background and border are not clipped. Adorners respect rounded clips too.
      </TextBlock>

      <StackPanel Spacing="8">
        <TextBlock Classes="h3">Border: Rounded Child Clip</TextBlock>
        <Border Height="72"
                Width="160"
                Background="Red"
                BorderBrush="Green"
                BorderThickness="2"
                CornerRadius="16"
                ClipToBounds="True">
          <Border Width="80"
                  Height="72"
                  Background="Blue"
                  HorizontalAlignment="Right" />
        </Border>
        <TextBlock Text="Only the child is clipped; the border/background remain intact" />
      </StackPanel>

      <StackPanel Spacing="8">
        <TextBlock Classes="h3">ContentPresenter: Rounded Child Clip</TextBlock>
        <ContentPresenter Width="160"
                          Height="72"
                          Background="Red"
                          BorderBrush="Green"
                          BorderThickness="2"
                          CornerRadius="16"
                          ClipToBounds="True"
                          HorizontalContentAlignment="Right">
          <ContentPresenter.Content>
            <Border Width="80"
                    Height="72"
                    Background="Blue" />
          </ContentPresenter.Content>
        </ContentPresenter>
        <TextBlock Text="Only the child is clipped; the presenter background remains intact" />
      </StackPanel>

      <StackPanel Spacing="8">
        <TextBlock Classes="h3">Adorner: Respects Rounded Clip</TextBlock>
        <Border Width="160"
                Height="160"
                Background="Transparent"
                BorderBrush="Black"
                BorderThickness="4"
                CornerRadius="24"
                ClipToBounds="True">
          <AdornerLayer.Adorner>
            <Border Background="DeepSkyBlue"
                    Opacity="0.6"
                    AdornerLayer.IsClipEnabled="True" />
          </AdornerLayer.Adorner>
        </Border>
        <TextBlock Text="Adorner is clipped to the rounded bounds of the adorned control" />
      </StackPanel>

      <StackPanel Spacing="8">
        <TextBlock Classes="h3">Geometry Child Clip (Layout Clip)</TextBlock>
        <pages:StarLayoutClipDecorator Width="200"
                                       Height="200"
                                       ClipToBounds="False">
          <Border Background="DodgerBlue" />
        </pages:StarLayoutClipDecorator>
        <TextBlock Text="GetLayoutClip affects only children; ClipToBounds is not required" />
      </StackPanel>
    </StackPanel>
  </ScrollViewer>
</UserControl>