Handle partial last items when items added.

Fixes #565.

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -105,6 +105,8 @@ namespace Avalonia.Controls.Presenters
         {
             base.ItemsChanged(items, e);
 
+            var panel = VirtualizingPanel;
+
             if (items != null)
             {
                 switch (e.Action)
@@ -147,6 +149,14 @@ namespace Avalonia.Controls.Presenters
                 FirstIndex = NextIndex = 0;
             }
 
+            // If we are scrolled to view a partially visible last item but controls were added
+            // then we need to return to a non-offset scroll position.
+            if (panel.PixelOffset != 0 && FirstIndex + panel.Children.Count < ItemCount)
+            {
+                panel.PixelOffset = 0;
+                RecycleContainersForMove(1);
+            }
+
             InvalidateScroll();
         }
 

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

@@ -456,6 +456,32 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(expected, actual);
         }
 
+        [Fact]
+        public void Scrolling_To_Partial_Last_Item_Then_Adding_Item_Updates_Containers()
+        {
+            var target = CreateTarget(itemCount: 10);
+            var items = (IList<string>)target.Items;
+
+            target.ApplyTemplate();
+            target.Measure(new Size(100, 95));
+            target.Arrange(new Rect(0, 0, 100, 95));
+
+            ((ILogicalScrollable)target).Offset = new Vector(0, 1);
+            Assert.Equal(new Vector(0, 1), ((ILogicalScrollable)target).Offset);
+
+            var expected = Enumerable.Range(0, 10).Select(x => $"Item {x}").ToList();
+            var actual = target.Panel.Children.Select(x => x.DataContext).ToList();
+            Assert.Equal(expected, actual);
+            Assert.Equal(10, ((IVirtualizingPanel)target.Panel).PixelOffset);
+
+            items.Add("Item 10");
+
+            expected = Enumerable.Range(1, 10).Select(x => $"Item {x}").ToList();
+            actual = target.Panel.Children.Select(x => x.DataContext).ToList();
+            Assert.Equal(expected, actual);
+            Assert.Equal(0, ((IVirtualizingPanel)target.Panel).PixelOffset);
+        }
+
         public class Vertical
         {
             [Fact]