Add unit test for `UseLayoutRounding`.

tests/Avalonia.Layout.UnitTests/LayoutableTests.cs

@@ -1,5 +1,6 @@
 using System;
 using Avalonia.Controls;
+using Avalonia.UnitTests;
 using Moq;
 using Xunit;
 
@@ -171,6 +172,37 @@ namespace Avalonia.Layout.UnitTests
             target.Verify(x => x.InvalidateMeasure(root), Times.Once());
         }
 
+        [Theory]
+        [InlineData(16, 6, 5.333333333333333)]
+        [InlineData(18, 10, 4)]
+        public void UseLayoutRounding_Arranges_Center_Alignment_Correctly_With_Fractional_Scaling(
+            double containerWidth,
+            double childWidth,
+            double expectedX)
+        {
+            Border target;
+            var root = new TestRoot
+            {
+                LayoutScaling = 1.5,
+                UseLayoutRounding = true,
+                Child = new Decorator
+                {
+                    Width = containerWidth,
+                    Height = 100,
+                    Child = target = new Border
+                    {
+                        Width = childWidth,
+                        HorizontalAlignment = HorizontalAlignment.Center,
+                    }
+                }
+            };
+
+            root.Measure(new Size(100, 100));
+            root.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(expectedX, 0, childWidth, 100), target.Bounds);
+        }
+
         private class TestLayoutable : Layoutable
         {
             public Size ArrangeSize { get; private set; }

tests/Avalonia.UnitTests/TestRoot.cs

@@ -42,7 +42,7 @@ namespace Avalonia.UnitTests
 
         public Size MaxClientSize { get; set; } = Size.Infinity;
 
-        public double LayoutScaling => 1;
+        public double LayoutScaling { get; set; } = 1;
 
         public ILayoutManager LayoutManager { get; set; } = new LayoutManager();