fix tests and and fixes.

src/Avalonia.Controls/Border.cs

@@ -110,9 +110,8 @@ namespace Avalonia.Controls
         {
             if (Child != null)
             {
-                var padding = Padding + BorderThickness;
-                var arrangeRect = new Rect(finalSize).Deflate(padding);
-                Child.Arrange(new Rect(arrangeRect.X, arrangeRect.Y, Math.Max(0, arrangeRect.Width), Math.Max(0, arrangeRect.Height)));                            
+                var padding = Padding + BorderThickness;                
+                Child.Arrange(new Rect(finalSize).Deflate(padding));
             }
 
             _borderRenderHelper.Update(finalSize, BorderThickness, CornerRadius);           

tests/Avalonia.Controls.UnitTests/BorderTests.cs

@@ -31,17 +31,16 @@ namespace Avalonia.Controls.UnitTests
                 Padding = new Thickness(6),
                 MaxHeight = 12,
                 MaxWidth = 12,
-                Content = content = new Border
+                Child = content = new Border
                 {
                      Height = 0,
                      Width = 0
                 }
             };
-
-            target.Measure(new Size(100, 100));
+            
             target.Arrange(new Rect(0, 0, 100, 100));
 
-            Assert.Equal(new Rect(0, 0, 0, 0), content.Bounds);
+            Assert.Equal(new Rect(6, 6, 0, 0), content.Bounds);
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Layout.cs

@@ -217,21 +217,23 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Border content;
             var target = new ContentPresenter
             {
-                Padding = 32,
+                Padding = new Thickness(32),
                 MaxHeight = 32,
                 MaxWidth = 32,
+                HorizontalContentAlignment = HorizontalAlignment.Center,
+                VerticalContentAlignment = VerticalAlignment.Center,
                 Content = content = new Border
                 {
-                    MinWidth = 16,
-                    MinHeight = 16,                    
+                    Height = 0,
+                    Width = 0,
                 },
             };
 
             target.UpdateChild();
-            target.Measure(new Size(100, 100));
+
             target.Arrange(new Rect(0, 0, 100, 100));
 
-            Assert.Equal(new Rect(0, 0, 0, 0), content.Bounds);
+            Assert.Equal(new Rect(48, 48, 0, 0), content.Bounds);
         }
     }
 }