Simplified test.

Simplified @donandren's test for #1936.

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.ObjectModel;
 using System.Linq;
+using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
@@ -171,106 +172,28 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(new Size(100, 10), target.Scroll.Viewport);
         }
 
-        [Theory]
-        [InlineData(ItemVirtualizationMode.Simple)]
-        [InlineData(ItemVirtualizationMode.None)]
-        public void When_Added_Removed_AfterItems_Reset_Should_Work(ItemVirtualizationMode virtMode)
+        [Fact]
+        public void Containers_Correct_After_Clear_Add_Remove()
         {
-            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            // Issue #1936
+            var items = new AvaloniaList<string>(Enumerable.Range(0, 11).Select(x => $"Item {x}"));
+            var target = new ListBox
             {
-                var items = new ObservableCollection<string>();
-
-                void create()
-                {
-                    foreach (var i in Enumerable.Range(1, 7))
-                    {
-                        items.Add(i.ToString());
-                    }
-                }
-
-                create();
-
-                var wnd = new Window() { SizeToContent = SizeToContent.WidthAndHeight };
-
-                wnd.IsVisible = true;
-
-                var target = new ListBox() { VirtualizationMode = virtMode };
-
-                wnd.Content = target;
-
-                var lm = wnd.LayoutManager;
-
-                target.Height = 110;//working fine when <=106 or >=119
-                target.Width = 50;
-
-                target.ItemTemplate = new FuncDataTemplate<object>(c =>
-                {
-                    var tb = new TextBlock() { Height = 10, Width = 30 };
-                    tb.Bind(TextBlock.TextProperty, new Binding());
-                    return tb;
-                }, true);
-
-                target.DataContext = items;
-
-                lm.ExecuteInitialLayoutPass(wnd);
-
-                target.Bind(ItemsControl.ItemsProperty, new Binding());
-
-                lm.ExecuteLayoutPass();
-
-                var panel = target.Presenter.Panel;
-
-                string itemsToString() =>
-                      string.Join(",", panel.Children.OfType<ListBoxItem>().Select(l => l.Content.ToString()).ToArray());
-
-                void addafter(string item, string newitem)
-                {
-                    items.Insert(items.IndexOf(item) + 1, newitem);
-                    lm.ExecuteLayoutPass();
-                }
-
-                void remove(string item)
-                {
-                    items.Remove(item);
-                    lm.ExecuteLayoutPass();
-                }
-
-                addafter("1", "1+");//expected 1,1+,2,3,4,5,6,7
-
-                addafter("2", "2+");//expected 1,1+,2,2+,3,4,5,6
-
-                remove("2+");//expected 1,1+,2,3,4,5,6,7
-
-                //Reset items
-                items.Clear();
-                create();
-
-                addafter("1", "1+");//expected 1,1+,2,3,4,5,6,7
-
-                addafter("2", "2+");//expected 1,1+,2,2+,3,4,5,6
-
-                remove("2+");//expected 1,1+,2,3,4,5,6,7
-
-                var sti = itemsToString();
-
-                var lbItems = panel.Children.OfType<ListBoxItem>().ToArray();
-
-                Assert.Equal("1", lbItems[0].Content);
-                Assert.Equal("1+", lbItems[1].Content);
-                Assert.Equal("2", lbItems[2].Content);
-                Assert.Equal("3", lbItems[3].Content); //bug it's 2+ instead
-                Assert.Equal("4", lbItems[4].Content);
+                Template = ListBoxTemplate(),
+                Items = items,
+                ItemTemplate = new FuncDataTemplate<string>(x => new TextBlock { Width = 20, Height = 10 }),
+                SelectedIndex = 0,
+            };
 
-                int lbi = 0;
+            Prepare(target);
 
-                //ensure all items are fine
-                foreach (var lb in lbItems)
-                {
-                    Assert.Equal(items[lbi++], lb.Content);
-                }
+            items.Clear();
+            items.AddRange(Enumerable.Range(0, 11).Select(x => $"Item {x}"));
+            items.Remove("Item 2");
 
-                //Assert.Equal("1,1+,2,3,4,5,6,7", sti);
-            }
+            Assert.Equal(
+                items,
+                target.Presenter.Panel.Children.Cast<ListBoxItem>().Select(x => (string)x.Content));
         }
 
         private FuncControlTemplate ListBoxTemplate()