Avalonia/samples/RenderDemo/Pages/ClippingPage.xaml

<UserControl 
  xmlns="https://github.com/avaloniaui" 
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  x:Class="RenderDemo.Pages.ClippingPage">
  <Grid ColumnDefinitions="Auto" RowDefinitions="Auto,Auto">
    <Grid.Styles>
      <Styles>
        <Style Selector="Border#clipped:pointerover">
          <Setter Property="Border.Background" Value="Crimson"/>
        </Style>
        <Style Selector="Border#clipChild">
          <Style.Animations>
            <Animation Duration="0:0:2" IterationCount="Infinite">
              <KeyFrame Cue="100%">
                <Setter Property="RotateTransform.Angle" Value="360"/>
              </KeyFrame>
            </Animation>
          </Style.Animations>
        </Style>
      </Styles>
    </Grid.Styles>
    <Border Name="clipped" 
            Background="Yellow" 
            Width="100" 
            Height="100" 
            Clip="M 58.625 0.07421875
                  C 50.305778 0.26687364 42.411858 7.0346526 41.806641 15.595703
                  C 42.446442 22.063923 39.707425 13.710754 36.982422 12.683594
                  C 29.348395 6.1821635 16.419398 8.4359222 11.480469 17.195312
                  C 6.0935256 25.476803 9.8118851 37.71125 18.8125 41.6875
                  C 9.1554771 40.62945 -0.070876925 49.146842 0.21679688 58.857422 
                  C 0.21545578 60.872512 0.56758794 62.88911 1.2617188 64.78125 
                  C 4.3821886 74.16708 16.298268 78.921772 25.03125 74.326172 
                  C 28.266843 72.062552 26.298191 74.214838 25.414062 76.398438
                  C 21.407348 85.589198 27.295992 97.294293 37.097656 99.501953 
                  C 46.864883 102.3541 57.82177 94.726518 58.539062 84.580078 
                  C 58.142158 79.498998 59.307538 83.392694 61.207031 85.433594 
                  C 67.532324 93.056874 80.440232 93.192029 86.882812 85.630859 
                  C 93.836392 78.456939 92.396838 65.538666 84.115234 60.009766 
                  C 79.783641 57.904836 83.569793 58.802369 86.375 58.193359 
                  C 96.383335 56.457569 102.87506 44.824101 99.083984 35.394531 
                  C 95.963498 26.008711 84.047451 21.254079 75.314453 25.849609
                  C 72.078834 28.113269 74.047517 25.960974 74.931641 23.777344 
                  C 78.93827 14.586564 73.049722 2.8815081 63.248047 0.67382812
                  C 61.721916 0.22817968 60.165597 0.038541919 58.625 0.07421875 z ">
      <Border Name="clipChild" Background="{DynamicResource ThemeAccentBrush}" Margin="4">
        <!-- Setting opacity puts the TextBox on a new layer -->
        <TextBox Text="Avalonia" Opacity="0.9" VerticalAlignment="Center"/>
        <Border.RenderTransform>
          <RotateTransform/>
        </Border.RenderTransform>
      </Border>
    </Border>
    <CheckBox Name="useMask" IsChecked="True" Grid.Row="1">Apply Geometry Clip</CheckBox>
  </Grid>
</UserControl>