Fixed ScrollPresenter arrange.

And added tests because I think it's about the 5th time I've tried to
get this right...

Perspex.Controls/Presenters/ScrollContentPresenter.cs

@@ -95,7 +95,7 @@ namespace Perspex.Controls.Presenters
 
             if (child != null)
             {
-                child.Arrange(new Rect((Point)(-this.Offset), child.DesiredSize));
+                child.Arrange(new Rect((Point)(-this.Offset), finalSize));
                 return finalSize;
             }
 

Tests/Perspex.Controls.UnitTests/ScrollContentPresenterTests.cs

@@ -11,9 +11,144 @@ namespace Perspex.Controls.UnitTests
     using System.Reactive.Linq;
     using Perspex.Controls.Presenters;
     using Xunit;
+    using Perspex.Layout;
 
     public class ScrollContentPresenterTests
     {
+        [Fact]
+        public void Content_Can_Be_Left_Aligned()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Padding = new Thickness(8),
+                    HorizontalAlignment = HorizontalAlignment.Left
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(0, 0, 16, 100), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_Stretched()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Padding = new Thickness(8),
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(0, 0, 100, 100), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_Right_Aligned()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Padding = new Thickness(8),
+                    HorizontalAlignment = HorizontalAlignment.Right
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(84, 0, 16, 100), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_Bottom_Aligned()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Padding = new Thickness(8),
+                    VerticalAlignment = VerticalAlignment.Bottom,
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(0, 84, 100, 16), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_TopRight_Aligned()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Padding = new Thickness(8),
+                    HorizontalAlignment = HorizontalAlignment.Right,
+                    VerticalAlignment = VerticalAlignment.Top,
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(84, 0, 16, 16), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_Larger_Than_Viewport()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Width = 150,
+                    Height = 150,
+                }),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(0, 0, 150, 150), content.Bounds);
+        }
+
+        [Fact]
+        public void Content_Can_Be_Offset()
+        {
+            Border content;
+            var target = new ScrollContentPresenter
+            {
+                Content = (content = new Border
+                {
+                    Width = 150,
+                    Height = 150,
+                }),
+                Offset = new Vector(25, 25),
+            };
+
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Rect(-25, -25, 150, 150), content.Bounds);
+        }
+
         [Fact]
         public void Arrange_Should_Set_Viewport_And_Extent_In_That_Order()
         {

Tests/Perspex.Controls.UnitTests/ScrollViewerTests.cs

@@ -13,6 +13,7 @@ namespace Perspex.Controls.UnitTests
     using Perspex.Controls.Templates;
     using Perspex.VisualTree;
     using Xunit;
+    using Perspex.Layout;
 
     public class ScrollViewerTests
     {