Allow changing orientation in virt sample.

samples/VirtualizationTest/MainWindow.xaml

@@ -7,6 +7,8 @@
                     Gap="4">
             <DropDown Items="{Binding VirtualizationModes}"
                       SelectedItem="{Binding VirtualizationMode}"/>
+            <DropDown Items="{Binding Orientations}"
+                      SelectedItem="{Binding Orientation}"/>
             <TextBox Watermark="Item Count"
                      UseFloatingWatermark="True"
                      Text="{Binding ItemCount}"/>
@@ -32,6 +34,11 @@
                  SelectedItems="{Binding SelectedItems}"
                  SelectionMode="Multiple"
                  VirtualizationMode="{Binding VirtualizationMode}">
+            <ListBox.ItemsPanel>
+                <ItemsPanelTemplate>
+                    <VirtualizingStackPanel Orientation="{Binding Orientation}"/>
+                </ItemsPanelTemplate>
+            </ListBox.ItemsPanel>
             <ListBox.ItemTemplate>
                 <DataTemplate>
                     <TextBlock Text="{Binding Header}"/>

samples/VirtualizationTest/ViewModels/MainWindowViewModel.cs

@@ -17,6 +17,7 @@ namespace VirtualizationTest.ViewModels
         private int _newItemIndex;
         private IReactiveList<ItemViewModel> _items;
         private string _prefix = "Item";
+        private Orientation _orientation;
         private ItemVirtualizationMode _virtualizationMode = ItemVirtualizationMode.Simple;
 
         public MainWindowViewModel()
@@ -53,6 +54,15 @@ namespace VirtualizationTest.ViewModels
             private set { this.RaiseAndSetIfChanged(ref _items, value); }
         }
 
+        public Orientation Orientation
+        {
+            get { return _orientation; }
+            set { this.RaiseAndSetIfChanged(ref _orientation, value); }
+        }
+
+        public IEnumerable<Orientation> Orientations =>
+            Enum.GetValues(typeof(Orientation)).Cast<Orientation>();
+
         public ItemVirtualizationMode VirtualizationMode
         {
             get { return _virtualizationMode; }

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -72,7 +72,10 @@ namespace Avalonia.Controls.Presenters
                         // offset the panel by the height of the first item.
                         var firstIndex = ItemCount - panel.Children.Count;
                         RecycleContainersForMove(firstIndex - FirstIndex);
-                        panel.PixelOffset = panel.Children[0].Bounds.Height;
+
+                        panel.PixelOffset = VirtualizingPanel.ScrollDirection == Orientation.Vertical ?
+                            panel.Children[0].Bounds.Height :
+                            panel.Children[0].Bounds.Width;
                     }
                 }
             }