Fix MeasureCore when there's a size constraint and there's a negative margin. (#18462)

src/Avalonia.Base/Layout/Layoutable.cs

@@ -596,15 +596,15 @@ namespace Avalonia.Layout
                     (width, height) = LayoutHelper.RoundLayoutSizeUp(new Size(width, height), scale);
                 }
 
+                width += margin.Left + margin.Right;
+                height += margin.Top + margin.Bottom;
+
                 if (width > availableSize.Width)
                     width = availableSize.Width;
 
                 if (height > availableSize.Height)
                     height = availableSize.Height;
 
-                width += margin.Left + margin.Right;
-                height += margin.Top + margin.Bottom;
-
                 if (width < 0)
                     width = 0;
 

tests/Avalonia.Base.UnitTests/Layout/LayoutableTests.cs

@@ -435,6 +435,35 @@ namespace Avalonia.Base.UnitTests.Layout
             });
         }
 
+        [Fact]
+        public void Constraint_And_Negative_Margin()
+        {
+            using var app = UnitTestApplication.Start(TestServices.MockPlatformRenderInterface);
+
+            var textBlock = new TextBlock
+            {
+                Margin = new Thickness(-10),
+                Text = "Lorem ipsum dolor sit amet",
+            };
+
+            var border = new Border
+            {
+                MaxWidth = 100,
+                Child = textBlock,
+            };
+
+            border.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
+            border.Arrange(new Rect(default, border.DesiredSize));
+
+            Assert.Multiple(() =>
+            {
+                Assert.Equal(new Size(100, 0), border.DesiredSize);
+                Assert.Equal(new Rect(0, 0, 100, 0), border.Bounds);
+                Assert.Equal(new Size(100, 0), textBlock.DesiredSize);
+                Assert.Equal(new Rect(-10, -10, 120, 20), textBlock.Bounds);
+            });
+        }
+
         private class TestLayoutable : Layoutable
         {
             public Size ArrangeSize { get; private set; }