Updated overflow algorithm.

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -41,7 +41,7 @@ namespace Avalonia.Controls
             {
                 var bounds = Orientation == Orientation.Horizontal ? 
                     Bounds.Width : Bounds.Height;
-                return Math.Max(0, (_takenSpace - _pixelOffset) - bounds);
+                return Math.Max(0, _takenSpace - bounds);
             }
         }
 
@@ -82,6 +82,7 @@ namespace Avalonia.Controls
             _averageItemSize = 0;
             _averageCount = 0;
             var result = base.ArrangeOverride(finalSize);
+            _takenSpace += _pixelOffset;
             Controller?.UpdateControls();
             return result;
         }

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -80,6 +80,52 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Equal(2, target.OverflowCount);
             }
 
+            [Fact]
+            public void Reports_PixelOverflow_After_Arrange()
+            {
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 52 });
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(2, target.PixelOverflow);
+            }
+
+            [Fact]
+            public void Reports_PixelOverflow_With_PixelOffset()
+            {
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 52 });
+                target.PixelOffset = 2;
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(2, target.PixelOverflow);
+            }
+
+            [Fact]
+            public void PixelOffset_Can_Be_More_Than_Child_Without_Affecting_IsFull()
+            {
+                var target = (IVirtualizingPanel)new VirtualizingStackPanel();
+
+                target.Children.Add(new Canvas { Width = 50, Height = 50 });
+                target.Children.Add(new Canvas { Width = 50, Height = 52 });
+                target.PixelOffset = 55;
+
+                target.Measure(new Size(100, 100));
+                target.Arrange(new Rect(target.DesiredSize));
+
+                Assert.Equal(55, target.PixelOffset);
+                Assert.Equal(2, target.PixelOverflow);
+                Assert.True(target.IsFull);
+            }
+
             [Fact]
             public void Passes_Navigation_Request_To_ILogicalScrollable_Parent()
             {