fix repro.

samples/ControlCatalog/MainWindow.xaml

@@ -9,7 +9,7 @@
             </Style>
         </Grid.Styles>
         <local:MyControl Height="200" Width="200" Background="Red">
-            <Button local:MyControl.Proportion="0.2" />
+            <Button />
         </local:MyControl>
     </Grid>
 </Window>

samples/ControlCatalog/MyControl.cs

@@ -13,7 +13,7 @@ namespace ControlCatalog
         /// Defines the Proportion attached property.
         /// </summary>
         public static readonly AttachedProperty<double> ProportionProperty =
-            AvaloniaProperty.RegisterAttached<MyControl, IControl, double>("Proportion", double.NaN, defaultBindingMode: Avalonia.Data.BindingMode.TwoWay);
+            AvaloniaProperty.RegisterAttached<MyControl, Control, double>("Proportion", double.NaN, defaultBindingMode: Avalonia.Data.BindingMode.TwoWay);
 
         /// <summary>
         /// Gets the value of the Proportion attached property on the specified control.