Complete

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -12,6 +12,11 @@
     </Style>
     <Style Selector="ItemsRepeater TextBlock:nth-child(5n+3)">
       <Setter Property="Foreground" Value="Red" />
+      <Setter Property="FontWeight" Value="Bold" />
+    </Style>
+    <Style Selector="ItemsRepeater TextBlock:nth-last-child(5n+4)">
+      <Setter Property="Foreground" Value="Blue" />
+      <Setter Property="FontWeight" Value="Bold" />
     </Style>
   </UserControl.Styles>
   <UserControl.Resources>

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -3,8 +3,13 @@
              x:Class="ControlCatalog.Pages.ListBoxPage">
   <DockPanel>
     <DockPanel.Styles>
-      <Style Selector="ListBox ListBoxItem:nth-child(2n)">
+      <Style Selector="ListBox ListBoxItem:nth-child(5n+3)">
+        <Setter Property="Foreground" Value="Red" />
+        <Setter Property="FontWeight" Value="Bold" />
+      </Style>
+      <Style Selector="ListBox ListBoxItem:nth-last-child(5n+4)">
         <Setter Property="Foreground" Value="Blue" />
+        <Setter Property="FontWeight" Value="Bold" />
       </Style>
     </DockPanel.Styles>
     <StackPanel DockPanel.Dock="Top" Margin="4">

src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections;
 using System.Collections.Specialized;
-using System.Linq;
 
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
@@ -133,7 +132,7 @@ namespace Avalonia.Controls.Presenters
 
         protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
 
-        int? IChildIndexProvider.TotalCount => Items.TryGetCountFast(out var count) ? count : null;
+        int? IChildIndexProvider.TotalCount => Items.TryGetCountFast(out var count) ? count : (int?)null;
 
         event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
         {
@@ -161,6 +160,8 @@ namespace Avalonia.Controls.Presenters
             if (Panel != null)
             {
                 ItemsChanged(e);
+
+                _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs());
             }
         }
 
@@ -192,7 +193,7 @@ namespace Avalonia.Controls.Presenters
         {
             for (var i = 0; i < e.Containers.Count; i++)
             {
-                _childIndexChanged?.Invoke(sender, new ChildIndexChangedEventArgs(e.Containers[i].ContainerControl));
+                _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(e.Containers[i].ContainerControl));
             }
         }
 

src/Avalonia.Styling/LogicalTree/ChildIndexChangedEventArgs.cs

@@ -3,6 +3,9 @@ using System;
 
 namespace Avalonia.LogicalTree
 {
+    /// <summary>
+    /// Event args for <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
+    /// </summary>
     public class ChildIndexChangedEventArgs : EventArgs
     {
         public ChildIndexChangedEventArgs()

src/Avalonia.Styling/LogicalTree/IChildIndexProvider.cs

@@ -3,12 +3,30 @@ using System;
 
 namespace Avalonia.LogicalTree
 {
+    /// <summary>
+    /// Child's index and total count information provider used by list-controls (ListBox, StackPanel, etc.)
+    /// </summary>
+    /// <remarks>
+    /// Used by nth-child and nth-last-child selectors. 
+    /// </remarks>
     public interface IChildIndexProvider
     {
+        /// <summary>
+        /// Gets child's actual index in order of the original source.
+        /// </summary>
+        /// <param name="child">Logical child.</param>
+        /// <returns>Index or -1 if child was not found.</returns>
         int GetChildIndex(ILogical child);
 
+        /// <summary>
+        /// Total children count or null if source is infinite.
+        /// Some Avalonia features might not work if <see cref="TotalCount"/> is null, for instance: nth-last-child selector.
+        /// </summary>
         int? TotalCount { get; }
 
+        /// <summary>
+        /// Notifies subscriber when child's index or total count was changed.
+        /// </summary>
         event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
     }
 }

src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs

@@ -44,7 +44,12 @@ namespace Avalonia.Styling.Activators
 
         private void ChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
         {
-            if (e.Child is null
+            // Run matching again if:
+            // 1. Selector is reversed, so other item insertion/deletion might affect total count without changing subscribed item index.
+            // 2. e.Child is null, when all children indeces were changed.
+            // 3. Subscribed child index was changed.
+            if (_reversed
+                || e.Child is null                
                 || e.Child == _control)
             {
                 PublishNext(IsMatching());

src/Avalonia.Styling/Styling/NthChildSelector.cs

@@ -7,6 +7,12 @@ using Avalonia.Styling.Activators;
 
 namespace Avalonia.Styling
 {
+    /// <summary>
+    /// The :nth-child() pseudo-class matches elements based on their position in a group of siblings.
+    /// </summary>
+    /// <remarks>
+    /// Element indices are 1-based.
+    /// </remarks>
     public class NthChildSelector : Selector
     {
         private const string NthChildSelectorName = "nth-child";
@@ -22,6 +28,12 @@ namespace Avalonia.Styling
             _reversed = reversed;
         }
 
+        /// <summary>
+        /// Creates an instance of <see cref="NthChildSelector"/>
+        /// </summary>
+        /// <param name="previous">Previous selector.</param>
+        /// <param name="step">Position step.</param>
+        /// <param name="offset">Initial index offset.</param>
         public NthChildSelector(Selector? previous, int step, int offset)
             : this(previous, step, offset, false)
         {

src/Avalonia.Styling/Styling/NthLastChildSelector.cs

@@ -2,8 +2,20 @@
 
 namespace Avalonia.Styling
 {
+    /// <summary>
+    /// The :nth-child() pseudo-class matches elements based on their position among a group of siblings, counting from the end.
+    /// </summary>
+    /// <remarks>
+    /// Element indices are 1-based.
+    /// </remarks>
     public class NthLastChildSelector : NthChildSelector
     {
+        /// <summary>
+        /// Creates an instance of <see cref="NthLastChildSelector"/>
+        /// </summary>
+        /// <param name="previous">Previous selector.</param>
+        /// <param name="step">Position step.</param>
+        /// <param name="offset">Initial index offset, counting from the end.</param>
         public NthLastChildSelector(Selector? previous, int step, int offset) : base(previous, step, offset, true)
         {
         }

src/Avalonia.Styling/Styling/Selectors.cs

@@ -123,11 +123,17 @@ namespace Avalonia.Styling
             return new NotSelector(previous, argument);
         }
 
+        /// <inheritdoc cref="NthChildSelector"/>
+        /// <inheritdoc cref="NthChildSelector(Selector?, int, int)"/>
+        /// <returns>The selector.</returns>
         public static Selector NthChild(this Selector previous, int step, int offset)
         {
             return new NthChildSelector(previous, step, offset);
         }
 
+        /// <inheritdoc cref="NthLastChildSelector"/>
+        /// <inheritdoc cref="NthLastChildSelector(Selector?, int, int)"/>
+        /// <returns>The selector.</returns>
         public static Selector NthLastChild(this Selector previous, int step, int offset)
         {
             return new NthLastChildSelector(previous, step, offset);

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/StyleTests.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Threading.Tasks;
@@ -336,6 +337,45 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             }
         }
 
+        [Fact]
+        public void Style_Can_Use_NthLastChild_Selector_After_Reoder()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Window.Styles>
+        <Style Selector='Border:nth-last-child(2n)'>
+            <Setter Property='Background' Value='Red'/>
+        </Style>
+    </Window.Styles>
+    <StackPanel x:Name='parent'>
+        <Border x:Name='b1' />
+        <Border x:Name='b2' />
+    </StackPanel>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+
+                var parent = window.FindControl<StackPanel>("parent");
+                var b1 = window.FindControl<Border>("b1");
+                var b2 = window.FindControl<Border>("b2");
+
+                Assert.Equal(Brushes.Red, b1.Background);
+                Assert.Null(b2.Background);
+
+                parent.Children.Remove(b1);
+
+                Assert.Null(b1.Background);
+                Assert.Null(b2.Background);
+
+                parent.Children.Add(b1);
+
+                Assert.Null(b1.Background);
+                Assert.Equal(Brushes.Red, b2.Background);
+            }
+        }
+
 
         [Fact]
         public void Style_Can_Use_NthChild_Selector_With_ListBox()
@@ -364,25 +404,18 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 
                 window.Show();
 
-                var items = list.Presenter.Panel.Children.Cast<ListBoxItem>();
-                ListBoxItem At(int index) => items.ElementAt(index);
+                IEnumerable<IBrush> GetColors() => list.Presenter.Panel.Children.Cast<ListBoxItem>().Select(t => t.Background);
 
-                Assert.Equal(Brushes.Transparent, At(0).Background);
-                Assert.Equal(Brushes.Green, At(1).Background);
-                Assert.Equal(Brushes.Transparent, At(2).Background);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Green, Brushes.Transparent }, GetColors());
 
                 collection.Remove(Brushes.Green);
 
-                Assert.Equal(Brushes.Transparent, At(0).Background);
-                Assert.Equal(Brushes.Blue, At(1).Background);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Blue }, GetColors());
 
                 collection.Add(Brushes.Violet);
                 collection.Add(Brushes.Black);
 
-                Assert.Equal(Brushes.Transparent, At(0).Background);
-                Assert.Equal(Brushes.Blue, At(1).Background);
-                Assert.Equal(Brushes.Transparent, At(2).Background);
-                Assert.Equal(Brushes.Black, At(3).Background);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Blue, Brushes.Transparent, Brushes.Black }, GetColors());
             }
         }
 
@@ -415,25 +448,19 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 
                 window.Show();
 
-                var items = list.Children;
-                TextBlock At(int index) => (TextBlock)list.GetOrCreateElement(index);
+                IEnumerable<IBrush> GetColors() => Enumerable.Range(0, list.ItemsSourceView.Count)
+                    .Select(t => (list.GetOrCreateElement(t) as TextBlock)!.Foreground);
 
-                Assert.Equal(Brushes.Transparent, At(0).Foreground);
-                Assert.Equal(Brushes.Green, At(1).Foreground);
-                Assert.Equal(Brushes.Transparent, At(2).Foreground);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Green, Brushes.Transparent }, GetColors());
 
                 collection.Remove(Brushes.Green);
 
-                Assert.Equal(Brushes.Transparent, At(0).Foreground);
-                Assert.Equal(Brushes.Blue, At(1).Foreground);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Blue }, GetColors());
 
                 collection.Add(Brushes.Violet);
                 collection.Add(Brushes.Black);
 
-                Assert.Equal(Brushes.Transparent, At(0).Foreground);
-                Assert.Equal(Brushes.Blue, At(1).Foreground);
-                Assert.Equal(Brushes.Transparent, At(2).Foreground);
-                Assert.Equal(Brushes.Black, At(3).Foreground);
+                Assert.Equal(new[] { Brushes.Transparent, Brushes.Blue, Brushes.Transparent, Brushes.Black }, GetColors());
             }
         }
 

tests/Avalonia.Styling.UnitTests/SelectorTests_NthChild.cs

@@ -196,7 +196,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public async Task Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
+        public void Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
         {
             Border b1;
             var contentControl = new ContentControl();
@@ -204,7 +204,7 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthChild(1, 0);
 
-            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.Equal(SelectorMatch.NeverThisInstance, target.Match(b1));
         }
 
         [Fact]

tests/Avalonia.Styling.UnitTests/SelectorTests_NthLastChild.cs

@@ -195,7 +195,7 @@ namespace Avalonia.Styling.UnitTests
         }
 
         [Fact]
-        public async Task Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
+        public void Nth_Child_Doesnt_Match_Control_Out_Of_Panel_Parent()
         {
             Border b1;
             var contentControl = new ContentControl();
@@ -203,7 +203,7 @@ namespace Avalonia.Styling.UnitTests
 
             var target = default(Selector).NthLastChild(1, 0);
 
-            Assert.False(await target.Match(b1).Activator!.Take(1));
+            Assert.Equal(SelectorMatch.NeverThisInstance, target.Match(b1));
         }
 
         [Fact]