Make ScrollContentViewer.Offset get coerced.

Use the same logic as for ScrollViewer.

src/Perspex.Controls/Presenters/ScrollContentPresenter.cs

@@ -28,6 +28,7 @@ namespace Perspex.Controls.Presenters
         static ScrollContentPresenter()
         {
             ClipToBoundsProperty.OverrideDefaultValue(typeof(ScrollContentPresenter), true);
+            OffsetProperty.OverrideValidation<ScrollContentPresenter>(ValidateOffset);
             AffectsArrange(OffsetProperty);
         }
 
@@ -143,5 +144,13 @@ namespace Perspex.Controls.Presenters
 
             Offset = offset;
         }
+
+        private static Vector ValidateOffset(ScrollContentPresenter o, Vector value)
+        {
+            return ScrollViewer.CoerceOffset(
+                o.GetValue(ExtentProperty),
+                o.GetValue(ViewportProperty),
+                value);
+        }
     }
 }

src/Perspex.Controls/ScrollViewer.cs

@@ -124,6 +124,13 @@ namespace Perspex.Controls
             set { SetValue(VerticalScrollBarVisibilityProperty, value); }
         }
 
+        internal static Vector CoerceOffset(Size extent, Size viewport, Vector offset)
+        {
+            var maxX = Math.Max(extent.Width - viewport.Width, 0);
+            var maxY = Math.Max(extent.Height - viewport.Height, 0);
+            return new Vector(Clamp(offset.X, 0, maxX), Clamp(offset.Y, 0, maxY));
+        }
+
         protected override Size MeasureOverride(Size availableSize)
         {
             return base.MeasureOverride(availableSize);
@@ -148,9 +155,7 @@ namespace Perspex.Controls
             {
                 var extent = scrollViewer.Extent;
                 var viewport = scrollViewer.Viewport;
-                var maxX = Math.Max(extent.Width - viewport.Width, 0);
-                var maxY = Math.Max(extent.Height - viewport.Height, 0);
-                return new Vector(Clamp(value.X, 0, maxX), Clamp(value.Y, 0, maxY));
+                return CoerceOffset(extent, viewport, value);
             }
             else
             {

tests/Perspex.Controls.UnitTests/Presenters/ScrollContentPresenterTests.cs

@@ -167,7 +167,7 @@ namespace Perspex.Controls.UnitTests.Presenters
         {
             var target = new ScrollContentPresenter
             {
-                Content = new Border { Width = 40, Height = 50 }
+                Content = new Border { Width = 140, Height = 150 }
             };
 
             target.Measure(new Size(100, 100));
@@ -178,6 +178,17 @@ namespace Perspex.Controls.UnitTests.Presenters
             Assert.False(target.IsArrangeValid);
         }
 
+        [Fact]
+        public void Offset_Should_Be_Coerced_To_Viewport()
+        {
+            var target = new ScrollContentPresenter();
+            target.SetValue(ScrollContentPresenter.ExtentProperty, new Size(20, 20));
+            target.SetValue(ScrollContentPresenter.ViewportProperty, new Size(10, 10));
+            target.Offset = new Vector(12, 12);
+
+            Assert.Equal(new Vector(10, 10), target.Offset);
+        }
+
         private class TestControl : Control
         {
             protected override Size MeasureOverride(Size availableSize)