Fixed ReversibleStackPanel not invalidating arrange on ReverseOrder change (#20172)

* Added unit tests for ReversibleStackPanel (#20171)
* Invalidates_Arrange_On_Reverse_Order_Change fails.

* Fixed ReversibleStackPanel not invalidating arrange on ReverseOrder change (#20171)

Author: Athari

src/Avalonia.Controls/Notifications/ReversibleStackPanel.cs

@@ -24,6 +24,11 @@ public bool ReverseOrder
             set => SetValue(ReverseOrderProperty, value);
         }
 
+        static ReversibleStackPanel()
+        {
+            AffectsArrange<ReversibleStackPanel>(ReverseOrderProperty);
+        }
+
         /// <inheritdoc/>
         protected override Size ArrangeOverride(Size finalSize)
         {

tests/Avalonia.Controls.UnitTests/ReversibleStackPanelTests.cs

@@ -0,0 +1,48 @@
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class ReversibleStackPanelTests : ScopedTestBase
+    {
+        [Fact]
+        public void Arranges_In_Reverse_Order()
+        {
+            var target = new ReversibleStackPanel
+            {
+                ReverseOrder = true,
+                Children =
+                {
+                    new Border { Height = 30, Width = 10 },
+                    new Border { Height = 50 },
+                }
+            };
+
+            target.Measure(Size.Infinity);
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(0, 50, 10, 30), target.Children[0].Bounds);
+            Assert.Equal(new Rect(0, 0, 10, 50), target.Children[1].Bounds);
+        }
+
+        [Fact]
+        public void Invalidates_Arrange_On_Reverse_Order_Change()
+        {
+            var target = new ReversibleStackPanel
+            {
+                Children =
+                {
+                    new Border(),
+                    new Border(),
+                }
+            };
+
+            target.Measure(Size.Infinity);
+            target.Arrange(new Rect(target.DesiredSize));
+            target.ReverseOrder = true;
+
+            Assert.True(target.IsMeasureValid);
+            Assert.False(target.IsArrangeValid);
+        }
+    }
+}