Fix off-by-one error in ItemContainerGenerator

Fixes #604.

src/Avalonia.Controls/Generators/ItemContainerGenerator.cs

@@ -228,7 +228,7 @@ namespace Avalonia.Controls.Generators
         /// <returns>The containers.</returns>
         protected IEnumerable<ItemContainerInfo> GetContainerRange(int index, int count)
         {
-            return _containers.Where(x => x.Key >= index && x.Key <= index + count).Select(x => x.Value);
+            return _containers.Where(x => x.Key >= index && x.Key < index + count).Select(x => x.Value);
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -31,7 +31,7 @@ namespace Avalonia.Controls.UnitTests
             ApplyTemplates(target);
 
             Assert.Equal(new[] { "Root" }, ExtractItemHeader(target, 0));
-            Assert.Equal(new[] { "Child1", "Child2" }, ExtractItemHeader(target, 1));
+            Assert.Equal(new[] { "Child1", "Child2", "Child3" }, ExtractItemHeader(target, 1));
             Assert.Equal(new[] { "Grandchild2a" }, ExtractItemHeader(target, 2));
         }
 
@@ -58,7 +58,7 @@ namespace Avalonia.Controls.UnitTests
                 .OfType<TreeViewItem>()
                 .ToList();
 
-            Assert.Equal(4, items.Count);
+            Assert.Equal(5, items.Count);
             Assert.All(items, x => Assert.IsType<Canvas>(x.HeaderPresenter.Child));
         }
 
@@ -199,11 +199,11 @@ namespace Avalonia.Controls.UnitTests
             root.Child = target;
             ApplyTemplates(target);
 
-            Assert.Equal(4, target.ItemContainerGenerator.Index.Items.Count());
+            Assert.Equal(5, target.ItemContainerGenerator.Index.Items.Count());
 
             tree[0].Children.RemoveAt(1);
 
-            Assert.Equal(2, target.ItemContainerGenerator.Index.Items.Count());
+            Assert.Equal(3, target.ItemContainerGenerator.Index.Items.Count());
         }
 
         [Fact]
@@ -297,7 +297,7 @@ namespace Avalonia.Controls.UnitTests
             ApplyTemplates(target);
 
             Assert.Equal(new[] { "Root" }, ExtractItemHeader(target, 0));
-            Assert.Equal(new[] { "Child1", "Child2" }, ExtractItemHeader(target, 1));
+            Assert.Equal(new[] { "Child1", "Child2", "Child3" }, ExtractItemHeader(target, 1));
             Assert.Equal(new[] { "Grandchild2a" }, ExtractItemHeader(target, 2));
 
             // Make sure that the binding to Node.Children does not get collected.
@@ -358,6 +358,10 @@ namespace Avalonia.Controls.UnitTests
                                 },
                             },
                         },
+                        new Node
+                        {
+                            Value = "Child3",
+                        },
                     }
                 }
             };