Add failing tests for #8866.

tests/Avalonia.Controls.UnitTests/BorderTests.cs

@@ -111,6 +111,57 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Equal(new Size(104, 104), target.DesiredSize);
             }
 
+            [Fact]
+            public void Measure_Arranges_Child_To_Rounded_BorderThickness()
+            {
+                Canvas child;
+                var target = new Border
+                {
+                    BorderThickness = new Thickness(1),
+                    Width = 82,
+                    Height = 82,
+                    Child = child = new Canvas(),
+                };
+
+                var root = CreateRoot(1.5, target);
+
+                root.LayoutManager.ExecuteInitialLayoutPass();
+
+                // - 1 pixel border thickness is rounded up to 1.3333; for both sides it is 2.6666
+                // - Size of 82 needs no rounding
+                // - Minus border thickness, space for child is 82 - 2.6666 = 79.3333
+                Assert.Equal(1.3333, child.Bounds.Left, 3);
+                Assert.Equal(1.3333, child.Bounds.Top, 3);
+                Assert.Equal(79.3333, child.Bounds.Width, 3);
+                Assert.Equal(79.3333, child.Bounds.Height, 3);
+            }
+
+            [Fact]
+            public void Measure_Arranges_Child_With_Rounded_Margin()
+            {
+                Border child;
+                var target = new Border
+                {
+                    Width = 220,
+                    Height = 220,
+                    Child = child = new Border
+                    {
+                        Margin = new Thickness(0, 25, 25, 25),
+                    },
+                };
+
+                var root = CreateRoot(1.5, target);
+
+                root.LayoutManager.ExecuteInitialLayoutPass();
+
+                // - 25 margin gets rounded up to 25.3333
+                // - Size of 220 needs no rounding
+                Assert.Equal(0, child.Bounds.Left, 3);
+                Assert.Equal(25.3333, child.Bounds.Top, 3);
+                Assert.Equal(194.6666, child.Bounds.Width, 3);
+                Assert.Equal(169.3333, child.Bounds.Height, 3);
+            }
+
             private static TestRoot CreateRoot(
                 double scaling,
                 Control child,