Fixed ScrollContentPresenter.

Did not react correctly to child changing.

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -59,6 +59,7 @@ namespace Avalonia.Controls.Presenters
         static ScrollContentPresenter()
         {
             ClipToBoundsProperty.OverrideDefaultValue(typeof(ScrollContentPresenter), true);
+            ChildProperty.Changed.AddClassHandler<ScrollContentPresenter>(x => x.ChildChanged);
             AffectsArrange(OffsetProperty);
         }
 
@@ -258,6 +259,16 @@ namespace Avalonia.Controls.Presenters
             e.Handled = BringDescendentIntoView(e.TargetObject, e.TargetRect);
         }
 
+        private void ChildChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            UpdateScrollableSubscription((IControl)e.NewValue);
+
+            if (e.OldValue != null)
+            {
+                Offset = default(Vector);
+            }
+        }
+
         private void UpdateScrollableSubscription(IControl child)
         {
             var scrollable = child as ILogicalScrollable;
@@ -286,12 +297,7 @@ namespace Avalonia.Controls.Presenters
             if (logicalScroll != scrollable.IsLogicalScrollEnabled)
             {
                 UpdateScrollableSubscription(Child);
-
-                if (!scrollable.IsLogicalScrollEnabled)
-                {
-                    Offset = default(Vector);
-                }
-
+                Offset = default(Vector);
                 InvalidateMeasure();
             }
 

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -109,7 +109,7 @@
     <Compile Include="ListBoxTests_Single.cs" />
     <Compile Include="NameScopeTests.cs" />
     <Compile Include="Primitives\SelectingItemsControlTests_Multiple.cs" />
-    <Compile Include="Presenters\ScrollContentPresenterTests_IScrollable.cs" />
+    <Compile Include="Presenters\ScrollContentPresenterTests_ILogicalScrollable.cs" />
     <Compile Include="TextBoxTests.cs" />
     <Compile Include="TextBlockTests.cs" />
     <Compile Include="WindowingPlatformMock.cs" />

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

@@ -236,6 +236,56 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Rect(0, 0, 100, 100), scrollable.Bounds);
         }
 
+        [Fact]
+        public void Changing_Content_Should_Update_State()
+        {
+            var logicalScrollable = new TestScrollable
+            {
+                Extent = new Size(100, 100),
+                Offset = new Vector(50, 50),
+                Viewport = new Size(25, 25),
+            };
+
+            var nonLogicalScrollable = new TestScrollable
+            {
+                IsLogicalScrollEnabled = false,
+            };
+
+            var target = new ScrollContentPresenter
+            {
+                Content = logicalScrollable,
+            };
+
+            target.UpdateChild();
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(logicalScrollable.Extent, target.Extent);
+            Assert.Equal(logicalScrollable.Offset, target.Offset);
+            Assert.Equal(logicalScrollable.Viewport, target.Viewport);
+            Assert.Equal(new Rect(0, 0, 100, 100), logicalScrollable.Bounds);
+
+            target.Content = nonLogicalScrollable;
+            target.UpdateChild();
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(new Size(150, 150), target.Extent);
+            Assert.Equal(new Vector(0, 0), target.Offset);
+            Assert.Equal(new Size(100, 100), target.Viewport);
+            Assert.Equal(new Rect(0, 0, 150, 150), nonLogicalScrollable.Bounds);
+
+            target.Content = logicalScrollable;
+            target.UpdateChild();
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(logicalScrollable.Extent, target.Extent);
+            Assert.Equal(logicalScrollable.Offset, target.Offset);
+            Assert.Equal(logicalScrollable.Viewport, target.Viewport);
+            Assert.Equal(new Rect(0, 0, 100, 100), logicalScrollable.Bounds);
+        }
+
         private class TestScrollable : Control, ILogicalScrollable
         {
             private Size _extent;