Merge pull request #8882 from AvaloniaUI/fixes/8866-border-child-rounding

Make layout rounding more resilient to floating point errors.

src/Avalonia.Base/Layout/LayoutHelper.cs

@@ -251,6 +251,17 @@ namespace Avalonia.Layout
         {
             double newValue;
 
+            // Round the value to avoid FP errors. This is needed because if `value` has a floating
+            // point precision error (e.g. 79.333333333333343) then when it's multiplied by
+            // `dpiScale` and rounded up, it will be rounded up to a value one greater than it
+            // should be.
+#if NET6_0_OR_GREATER
+            value = Math.Round(value, 8, MidpointRounding.ToZero);
+#else
+            // MidpointRounding.ToZero isn't available in netstandard2.0.
+            value = Math.Truncate(value * 1e8) / 1e8;
+#endif
+
             // If DPI == 1, don't use DPI-aware rounding.
             if (!MathUtilities.IsOne(dpiScale))
             {

src/Avalonia.Controls/Utils/BorderRenderHelper.cs

@@ -61,7 +61,7 @@ namespace Avalonia.Controls.Utils
                     _backgroundGeometryCache = null;
                 }
 
-                if (boundRect.Width != 0 && innerRect.Height != 0)
+                if (boundRect.Width != 0 && boundRect.Height != 0)
                 {
                     var borderGeometryKeypoints =
                         new BorderGeometryKeypoints(boundRect, borderThickness, cornerRadius, false);

tests/Avalonia.Controls.UnitTests/BorderTests.cs

@@ -78,7 +78,7 @@ namespace Avalonia.Controls.UnitTests
                     }
                 };
 
-                var root = CreatedRoot(1.5, target);
+                var root = CreateRoot(1.5, target);
 
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
@@ -101,7 +101,7 @@ namespace Avalonia.Controls.UnitTests
                     }
                 };
 
-                var root = CreatedRoot(1.5, target);
+                var root = CreateRoot(1.5, target);
 
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
@@ -111,7 +111,58 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Equal(new Size(104, 104), target.DesiredSize);
             }
 
-            private static TestRoot CreatedRoot(
+            [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,
                 Size? constraint = null)