Call AddOwner on Slider.Orientation instead of declaring a new property

src/Avalonia.Controls/Slider.cs

@@ -17,7 +17,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Orientation"/> property.
         /// </summary>
         public static readonly StyledProperty<Orientation> OrientationProperty =
-            AvaloniaProperty.Register<Slider, Orientation>(nameof(Orientation), Orientation.Horizontal);
+            ScrollBar.OrientationProperty.AddOwner<Slider>();
 
         /// <summary>
         /// Defines the <see cref="IsSnapToTickEnabled"/> property.
@@ -41,8 +41,7 @@ namespace Avalonia.Controls
         /// </summary>
         static Slider()
         {
-            PseudoClass(OrientationProperty, o => o == Avalonia.Controls.Orientation.Vertical, ":vertical");
-            PseudoClass(OrientationProperty, o => o == Avalonia.Controls.Orientation.Horizontal, ":horizontal");
+            OrientationProperty.OverrideDefaultValue(typeof(Slider), Orientation.Horizontal);
             Thumb.DragStartedEvent.AddClassHandler<Slider>(x => x.OnThumbDragStarted, RoutingStrategies.Bubble);
             Thumb.DragDeltaEvent.AddClassHandler<Slider>(x => x.OnThumbDragDelta, RoutingStrategies.Bubble);
             Thumb.DragCompletedEvent.AddClassHandler<Slider>(x => x.OnThumbDragCompleted, RoutingStrategies.Bubble);

tests/Avalonia.Controls.UnitTests/SliderTests.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class SliderTests
+    {
+        [Fact]
+        public void Default_Orientation_Should_Be_Horizontal()
+        {
+            var slider = new Slider();
+            Assert.Equal(Orientation.Horizontal, slider.Orientation);
+        }
+
+        [Fact]
+        public void Should_Set_Horizontal_Class()
+        {
+            var slider = new Slider
+            {
+                Orientation = Orientation.Horizontal
+            };
+
+            Assert.Contains(slider.Classes, ":horizontal".Equals);
+        }
+
+        [Fact]
+        public void Should_Set_Vertical_Class()
+        {
+            var slider = new Slider
+            {
+                Orientation = Orientation.Vertical
+            };
+
+            Assert.Contains(slider.Classes, ":vertical".Equals);
+        }
+    }
+}