add repro for binding to attached property from style.

samples/ControlCatalog/MainWindow.xaml

@@ -2,5 +2,10 @@
         Title="Avalonia Control Gallery"
         Icon="resm:ControlCatalog.Assets.test_icon.ico?assembly=ControlCatalog"
         xmlns:local="clr-namespace:ControlCatalog">
+    <Window.Styles>
+        <Style Selector="local|MainView">
+            <Setter Property="local:MyControl.Proportion" Value="{Binding Proportion}" />
+        </Style>
+    </Window.Styles>
     <local:MainView/>
-</Window>
+</Window>

samples/ControlCatalog/MainWindow.xaml.cs

@@ -13,6 +13,7 @@ namespace ControlCatalog
             this.AttachDevTools();
             //Renderer.DrawFps = true;
             //Renderer.DrawDirtyRects = Renderer.DrawFps = true;
+            DataContext = this;
         }
 
         private void InitializeComponent()
@@ -24,5 +25,14 @@ namespace ControlCatalog
             theme.TryGetResource("Button", out _);
             AvaloniaXamlLoader.Load(this);
         }
+
+        private double _proportion;
+
+        public double Proportion
+        {
+            get { return _proportion; }
+            set { _proportion = value; }
+        }
+
     }
 }

samples/ControlCatalog/MyControl.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Avalonia;
+using Avalonia.Controls;
+
+namespace ControlCatalog
+{
+    public class MyControl : Control
+    {
+        public static readonly AttachedProperty<double> ProportionProperty =
+            AvaloniaProperty.RegisterAttached<MyControl, IControl, double>("Proportion", double.NaN);
+
+        /// <summary>
+        /// Gets the value of the Proportion attached property on the specified control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <returns>The Proportion attached property.</returns>
+        public static double GetProportion(IControl control)
+        {
+            return control.GetValue(ProportionProperty);
+        }
+
+        /// <summary>
+        /// Sets the value of the Proportion attached property on the specified control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <param name="value">The value of the Dock property.</param>
+        public static void SetProportion(IControl control, double value)
+        {
+            control.SetValue(ProportionProperty, value);
+        }
+    }
+}