Make adding items work.

When virtualizing panel is not yet full.

src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs

@@ -110,10 +110,11 @@ namespace Avalonia.Controls.Presenters
                 switch (e.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
+                        CreateAndRemoveContainers();
+
                         if (e.NewStartingIndex >= FirstIndex &&
                             e.NewStartingIndex + e.NewItems.Count <= NextIndex)
                         {
-                            CreateAndRemoveContainers();
                             RecycleContainers();
                         }
 

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

@@ -62,6 +62,23 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(new Size(10, 0), ((ILogicalScrollable)target).Viewport);
         }
 
+        [Fact]
+        public void Should_Add_Containers_When_Panel_Is_Not_Full()
+        {
+            var target = CreateTarget(itemCount: 5);
+            var items = (IList<string>)target.Items;
+
+            target.ApplyTemplate();
+            target.Measure(new Size(100, 100));
+            target.Arrange(new Rect(0, 0, 100, 100));
+
+            Assert.Equal(5, target.Panel.Children.Count);
+
+            items.Add("New Item");
+
+            Assert.Equal(6, target.Panel.Children.Count);
+        }
+
         [Fact]
         public void Should_Remove_Items_When_Control_Is_Shrank()
         {