From 9de82b19393ff5ff4910637d99d15fb4d3f75294 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Thu, 19 Jan 2023 23:26:03 +0100
Subject: [PATCH 1/9] Make nth-last-child style change background.

Will hopefully make it a bit more obvious if it breaks in future: previously it was easier to miss as both `nth-child` and `nth-last-child` changed the foreground.
---
 samples/ControlCatalog/Pages/ListBoxPage.xaml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/samples/ControlCatalog/Pages/ListBoxPage.xaml b/samples/ControlCatalog/Pages/ListBoxPage.xaml
index dc0eaf0a51..b1b1b99c9c 100644
--- a/samples/ControlCatalog/Pages/ListBoxPage.xaml
+++ b/samples/ControlCatalog/Pages/ListBoxPage.xaml
@@ -10,7 +10,7 @@
         <Setter Property="FontWeight" Value="Bold" />
       </Style>
       <Style Selector="ListBox ListBoxItem:nth-last-child(5n+4)">
-        <Setter Property="Foreground" Value="Blue" />
+        <Setter Property="Background" Value="Blue" />
         <Setter Property="FontWeight" Value="Bold" />
       </Style>
     </DockPanel.Styles>

From 649e4fc57e45f4f8c357e61b7b66fd818eef9557 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 20 Jan 2023 10:57:40 +0100
Subject: [PATCH 2/9] Add failing test for #9997.

And a passing test for `nth-child` selector.
---
 .../VirtualizingStackPanelTests.cs            | 61 ++++++++++++++++++-
 1 file changed, 60 insertions(+), 1 deletion(-)

diff --git a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
index f1dd874c71..469f832b3a 100644
--- a/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
@@ -9,6 +9,8 @@ using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
+using Avalonia.Media;
+using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
 using Xunit;
@@ -278,6 +280,58 @@ namespace Avalonia.Controls.UnitTests
             Assert.Same(focused, target.GetRealizedElements().First());
         }
 
+        [Fact]
+        public void NthChild_Selector_Works()
+        {
+            using var app = App();
+            
+            var style = new Style(x => x.OfType<ContentPresenter>().NthChild(5, 0))
+            {
+                Setters = { new Setter(ListBoxItem.BackgroundProperty, Brushes.Red) },
+            };
+
+            var (target, _, _) = CreateTarget(styles: new[] { style });
+            var realized = target.GetRealizedContainers()!.Cast<ContentPresenter>().ToList();
+            
+            Assert.Equal(10, realized.Count);
+            
+            for (var i = 0; i < 10; ++i)
+            {
+                var container = realized[i];
+                var index = target.IndexFromContainer(container);
+                var expectedBackground = (i == 4 || i == 9) ? Brushes.Red : null;
+
+                Assert.Equal(i, index);
+                Assert.Equal(expectedBackground, container.Background);
+            }
+        }
+
+        [Fact]
+        public void NthLastChild_Selector_Works()
+        {
+            using var app = App();
+
+            var style = new Style(x => x.OfType<ContentPresenter>().NthLastChild(5, 0))
+            {
+                Setters = { new Setter(ListBoxItem.BackgroundProperty, Brushes.Red) },
+            };
+
+            var (target, _, _) = CreateTarget(styles: new[] { style });
+            var realized = target.GetRealizedContainers()!.Cast<ContentPresenter>().ToList();
+
+            Assert.Equal(10, realized.Count);
+
+            for (var i = 0; i < 10; ++i)
+            {
+                var container = realized[i];
+                var index = target.IndexFromContainer(container);
+                var expectedBackground = (i == 0 || i == 5) ? Brushes.Red : null;
+
+                Assert.Equal(i, index);
+                Assert.Equal(expectedBackground, container.Background);
+            }
+        }
+
         private static IReadOnlyList<int> GetRealizedIndexes(VirtualizingStackPanel target, ItemsControl itemsControl)
         {
             return target.GetRealizedElements()
@@ -322,7 +376,8 @@ namespace Avalonia.Controls.UnitTests
 
         private static (VirtualizingStackPanel, ScrollViewer, ItemsControl) CreateTarget(
             IEnumerable<object>? items = null,
-            bool useItemTemplate = true)
+            bool useItemTemplate = true,
+            IEnumerable<Style>? styles = null)
         {
             var target = new VirtualizingStackPanel();
 
@@ -351,6 +406,10 @@ namespace Avalonia.Controls.UnitTests
 
             var root = new TestRoot(true, itemsControl);
             root.ClientSize = new(100, 100);
+
+            if (styles is not null)
+                root.Styles.AddRange(styles);
+
             root.LayoutManager.ExecuteInitialLayoutPass();
 
             return (target, scroll, itemsControl);

From 2ef91ffc6d59cd563d99f342d3ade4d7115f10a7 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 20 Jan 2023 15:28:10 +0100
Subject: [PATCH 3/9] Fix new index calculation.

---
 src/Avalonia.Controls/VirtualizingStackPanel.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Avalonia.Controls/VirtualizingStackPanel.cs b/src/Avalonia.Controls/VirtualizingStackPanel.cs
index 3f539ce198..e556b1fc04 100644
--- a/src/Avalonia.Controls/VirtualizingStackPanel.cs
+++ b/src/Avalonia.Controls/VirtualizingStackPanel.cs
@@ -936,7 +936,7 @@ namespace Avalonia.Controls
 
                     // Update the indexes of the elements after the removed range.
                     end = _elements.Count;
-                    var newIndex = first;
+                    var newIndex = first + start;
                     for (var i = start; i < end; ++i)
                     {
                         if (_elements[i] is Control element)

From 8861f62ec175fd174eaeab780ab77f38fa48e78a Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 20 Jan 2023 15:47:28 +0100
Subject: [PATCH 4/9] Added index to ChildIndexChangedEventArgs.

---
 .../LogicalTree/ChildIndexChangedEventArgs.cs      | 12 +++++++++---
 .../Styling/Activators/NthChildActivator.cs        | 14 +++++++-------
 src/Avalonia.Base/Styling/NthChildSelector.cs      |  5 ++---
 .../Primitives/DataGridRowsPresenter.cs            |  2 +-
 src/Avalonia.Controls/ItemsControl.cs              |  4 ++--
 src/Avalonia.Controls/Repeater/ItemsRepeater.cs    |  9 +++++----
 6 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs b/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
index 497596fcc1..8f2be75e0f 100644
--- a/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
+++ b/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
@@ -12,17 +12,23 @@ namespace Avalonia.LogicalTree
 
         private ChildIndexChangedEventArgs()
         {
+            Index = -1;
         }
 
-        public ChildIndexChangedEventArgs(ILogical child)
+        public ChildIndexChangedEventArgs(ILogical child, int index)
         {
             Child = child;
+            Index = index;
         }
 
         /// <summary>
-        /// Logical child which index was changed.
-        /// If null, all children should be reset.
+        /// Gets the logical child whose index was changed or null if all children should be re-evaluated.
         /// </summary>
         public ILogical? Child { get; }
+
+        /// <summary>
+        /// Gets the new index of <see cref="Child"/> or -1 if all children should be re-evaluated.
+        /// </summary>
+        public int Index { get; }
     }
 }
diff --git a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
index 33d4cd0824..64aad9aa6b 100644
--- a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
+++ b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
@@ -13,6 +13,7 @@ namespace Avalonia.Styling.Activators
         private readonly int _step;
         private readonly int _offset;
         private readonly bool _reversed;
+        private int _index = -1;
 
         public NthChildActivator(
             ILogical control,
@@ -28,7 +29,8 @@ namespace Avalonia.Styling.Activators
 
         protected override bool EvaluateIsActive()
         {
-            return NthChildSelector.Evaluate(_control, _provider, _step, _offset, _reversed).IsMatch;
+            var index = _index >= 0 ? _index : _provider.GetChildIndex(_control);
+            return NthChildSelector.Evaluate(index, _provider, _step, _offset, _reversed).IsMatch;
         }
 
         protected override void Initialize() => _provider.ChildIndexChanged += ChildIndexChanged;
@@ -37,13 +39,11 @@ namespace Avalonia.Styling.Activators
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // 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 indices were changed.
-            // 3. Subscribed child index was changed.
-            if (_reversed
-                || e.Child is null
-                || e.Child == _control)
+            // 1. e.Child is null, when all children indices were changed.
+            // 2. Subscribed child index was changed.
+            if (e.Child is null || e.Child == _control)
             {
+                _index = e.Index;
                 ReevaluateIsActive();
             }
         }
diff --git a/src/Avalonia.Base/Styling/NthChildSelector.cs b/src/Avalonia.Base/Styling/NthChildSelector.cs
index ccfc2c781d..532179bb2c 100644
--- a/src/Avalonia.Base/Styling/NthChildSelector.cs
+++ b/src/Avalonia.Base/Styling/NthChildSelector.cs
@@ -61,7 +61,7 @@ namespace Avalonia.Styling
             {
                 return subscribe
                     ? new SelectorMatch(new NthChildActivator(logical, childIndexProvider, Step, Offset, _reversed))
-                    : Evaluate(logical, childIndexProvider, Step, Offset, _reversed);
+                    : Evaluate(childIndexProvider.GetChildIndex(logical), childIndexProvider, Step, Offset, _reversed);
             }
             else
             {
@@ -70,10 +70,9 @@ namespace Avalonia.Styling
         }
 
         internal static SelectorMatch Evaluate(
-            ILogical logical, IChildIndexProvider childIndexProvider,
+            int index, IChildIndexProvider childIndexProvider,
             int step, int offset, bool reversed)
         {
-            var index = childIndexProvider.GetChildIndex(logical);
             if (index < 0)
             {
                 return SelectorMatch.NeverThisInstance;
diff --git a/src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs b/src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs
index d906cd359c..5a4ddd36f4 100644
--- a/src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs
+++ b/src/Avalonia.Controls.DataGrid/Primitives/DataGridRowsPresenter.cs
@@ -66,7 +66,7 @@ namespace Avalonia.Controls.Primitives
 
         internal void InvalidateChildIndex(DataGridRow row)
         {
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(row, row.Index));
         }
 
         /// <summary>
diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index d19a04eb21..88991eec25 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -520,7 +520,7 @@ namespace Avalonia.Controls
         internal void ItemContainerPrepared(Control container, object? item, int index)
         {
             _containerBeingPrepared = new(index, container);
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container, index));
             _containerBeingPrepared = null;
 
             _scrollViewer?.RegisterAnchorCandidate(container);
@@ -529,7 +529,7 @@ namespace Avalonia.Controls
         internal void ItemContainerIndexChanged(Control container, int oldIndex, int newIndex)
         {
             ContainerIndexChangedOverride(container, oldIndex, newIndex);
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container, newIndex));
         }
 
         internal void ClearItemContainer(Control container)
diff --git a/src/Avalonia.Controls/Repeater/ItemsRepeater.cs b/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
index 4de6a5188d..deeda75c8b 100644
--- a/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
+++ b/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
@@ -557,11 +557,12 @@ namespace Avalonia.Controls
 
         internal void OnElementPrepared(Control element, VirtualizationInfo virtInfo)
         {
+            var index = virtInfo.Index;
+
             _viewportManager.OnElementPrepared(element, virtInfo);
 
             if (ElementPrepared != null)
             {
-                var index = virtInfo.Index;
 
                 if (_elementPreparedArgs == null)
                 {
@@ -575,7 +576,7 @@ namespace Avalonia.Controls
                 ElementPrepared(this, _elementPreparedArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, index));
         }
 
         internal void OnElementClearing(Control element)
@@ -594,7 +595,7 @@ namespace Avalonia.Controls
                 ElementClearing(this, _elementClearingArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, -1));
         }
 
         internal void OnElementIndexChanged(Control element, int oldIndex, int newIndex)
@@ -613,7 +614,7 @@ namespace Avalonia.Controls
                 ElementIndexChanged(this, _elementIndexChangedArgs);
             }
 
-            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
+            _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element, newIndex));
         }
 
         private void OnDataSourcePropertyChanged(ItemsSourceView? oldValue, ItemsSourceView? newValue)

From f289099aebd9fde11ecfe92df35a4303566ba85e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Fri, 20 Jan 2023 16:55:07 +0100
Subject: [PATCH 5/9] Added IChildIndexProvider.TotalCountChanged.

---
 .../LogicalTree/IChildIndexProvider.cs        |  7 ++++++-
 .../Styling/Activators/NthChildActivator.cs   | 19 +++++++++++++++++--
 src/Avalonia.Controls/ItemsControl.cs         |  8 ++++++++
 src/Avalonia.Controls/Panel.cs                | 15 +++++++++++++++
 4 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs b/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
index 7fcd73273c..a5e191d63e 100644
--- a/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
+++ b/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
@@ -25,8 +25,13 @@ namespace Avalonia.LogicalTree
         bool TryGetTotalCount(out int count);
 
         /// <summary>
-        /// Notifies subscriber when child's index or total count was changed.
+        /// Notifies subscriber when a child's index was changed.
         /// </summary>
         event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
+
+        /// <summary>
+        /// Notifies subscriber when the total child count changes.
+        /// </summary>
+        event EventHandler<EventArgs>? TotalCountChanged;
     }
 }
diff --git a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
index 64aad9aa6b..6c18081287 100644
--- a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
+++ b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
@@ -1,4 +1,5 @@
 #nullable enable
+using System;
 using Avalonia.LogicalTree;
 
 namespace Avalonia.Styling.Activators
@@ -33,8 +34,16 @@ namespace Avalonia.Styling.Activators
             return NthChildSelector.Evaluate(index, _provider, _step, _offset, _reversed).IsMatch;
         }
 
-        protected override void Initialize() => _provider.ChildIndexChanged += ChildIndexChanged;
-        protected override void Deinitialize() => _provider.ChildIndexChanged -= ChildIndexChanged;
+        protected override void Initialize()
+        {
+            _provider.ChildIndexChanged += ChildIndexChanged;
+            _provider.TotalCountChanged += TotalCountChanged;
+        }
+
+        protected override void Deinitialize()
+        {
+            _provider.ChildIndexChanged -= ChildIndexChanged;
+        }
 
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
@@ -47,5 +56,11 @@ namespace Avalonia.Styling.Activators
                 ReevaluateIsActive();
             }
         }
+
+        private void TotalCountChanged(object? sender, EventArgs e)
+        {
+            if (_reversed)
+                ReevaluateIsActive();
+        }
     }
 }
diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 88991eec25..6e89a4eb1d 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -88,6 +88,7 @@ namespace Avalonia.Controls
         private int _itemCount;
         private ItemContainerGenerator? _itemContainerGenerator;
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
+        private EventHandler<EventArgs>? _totalCountChanged;
         private IDataTemplate? _displayMemberItemTemplate;
         private Tuple<int, Control>? _containerBeingPrepared;
         private ScrollViewer? _scrollViewer;
@@ -203,6 +204,12 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
+        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
+        {
+            add => _totalCountChanged += value;
+            remove => _totalCountChanged -= value;
+        }
+
         /// <summary>
         /// Returns the container for the item at the specified index.
         /// </summary>
@@ -418,6 +425,7 @@ namespace Avalonia.Controls
             else if (change.Property == ItemCountProperty)
             {
                 UpdatePseudoClasses(change.GetNewValue<int>());
+                _totalCountChanged?.Invoke(this, EventArgs.Empty);
             }
             else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
             {
diff --git a/src/Avalonia.Controls/Panel.cs b/src/Avalonia.Controls/Panel.cs
index 007d18c813..ee18b3c4ab 100644
--- a/src/Avalonia.Controls/Panel.cs
+++ b/src/Avalonia.Controls/Panel.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.ComponentModel;
 using System.Linq;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
@@ -34,6 +35,7 @@ namespace Avalonia.Controls
         }
 
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
+        private EventHandler<EventArgs>? _totalCountChanged;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
@@ -41,6 +43,7 @@ namespace Avalonia.Controls
         public Panel()
         {
             Children.CollectionChanged += ChildrenChanged;
+            Children.PropertyChanged += ChildrenPropertyChanged;
         }
 
         /// <summary>
@@ -64,6 +67,12 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
+        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
+        {
+            add => _totalCountChanged += value;
+            remove => _totalCountChanged -= value;
+        }
+
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -161,6 +170,12 @@ namespace Avalonia.Controls
             InvalidateMeasure();
         }
 
+        private void ChildrenPropertyChanged(object? sender, PropertyChangedEventArgs e)
+        {
+            if (e.PropertyName == nameof(Children.Count) || e.PropertyName is null)
+                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+        }
+
         private static void AffectsParentArrangeInvalidate<TPanel>(AvaloniaPropertyChangedEventArgs e)
             where TPanel : Panel
         {

From a29bfdac72c0433e40e50a54c9a04d2c91d21651 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Sat, 21 Jan 2023 14:29:48 +0100
Subject: [PATCH 6/9] Clarify this code.

---
 .../Styling/Activators/NthChildActivator.cs         | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
index 6c18081287..e0fa056765 100644
--- a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
+++ b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
@@ -52,8 +52,21 @@ namespace Avalonia.Styling.Activators
             // 2. Subscribed child index was changed.
             if (e.Child is null || e.Child == _control)
             {
+                // We're using the _index field to pass the index of the child to EvaluateIsActive
+                // *only* when the active state is re-evaluated via this event handler. The docs
+                // for EvaluateIsActive say:
+                //
+                // > This method should read directly from its inputs and not rely on any
+                // > subscriptions to fire in order to be up-to-date.
+                //
+                // Which is good advice in general, however in this case we need to break the rule
+                // and use the value from the event subscription instead of calling
+                // IChildIndexProvider.GetChildIndex. This is because this event can be fired during
+                // the process of realizing an element of a virtualized list; in this case calling
+                // GetChildIndex may not return the correct index as the element isn't yet realized.
                 _index = e.Index;
                 ReevaluateIsActive();
+                _index = -1;
             }
         }
 

From b127453467de56a94524c9a10019e275612e1d86 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Sat, 21 Jan 2023 14:37:52 +0100
Subject: [PATCH 7/9] Remove hack from ItemsControl.

---
 src/Avalonia.Controls/ItemsControl.cs | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 6e89a4eb1d..fee394fe08 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -90,7 +90,6 @@ namespace Avalonia.Controls
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
         private EventHandler<EventArgs>? _totalCountChanged;
         private IDataTemplate? _displayMemberItemTemplate;
-        private Tuple<int, Control>? _containerBeingPrepared;
         private ScrollViewer? _scrollViewer;
 
         /// <summary>
@@ -527,10 +526,7 @@ namespace Avalonia.Controls
 
         internal void ItemContainerPrepared(Control container, object? item, int index)
         {
-            _containerBeingPrepared = new(index, container);
             _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(container, index));
-            _containerBeingPrepared = null;
-
             _scrollViewer?.RegisterAnchorCandidate(container);
         }
 
@@ -668,9 +664,6 @@ namespace Avalonia.Controls
 
         int IChildIndexProvider.GetChildIndex(ILogical child)
         {
-            if (_containerBeingPrepared?.Item2 == child)
-                return _containerBeingPrepared.Item1;
-
             return child is Control container ? IndexFromContainer(container) : -1;
         }
 

From d98467c3d87992b9d541679e3488bda282842c5e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 22 Feb 2023 17:48:49 +0100
Subject: [PATCH 8/9] Remove TotalCountChanged.

Merge it into existing `IChildIndexProvider.ChildIndexChanged` event.
---
 .../LogicalTree/ChildIndexChangedEventArgs.cs | 63 ++++++++++++++++---
 .../LogicalTree/IChildIndexProvider.cs        |  5 --
 .../Styling/Activators/NthChildActivator.cs   |  9 +--
 .../Primitives/DataGridCellsPresenter.cs      |  2 +-
 .../DataGridColumnHeadersPresenter.cs         |  2 +-
 src/Avalonia.Controls/ItemsControl.cs         | 11 +---
 src/Avalonia.Controls/Panel.cs                | 11 +---
 7 files changed, 65 insertions(+), 38 deletions(-)

diff --git a/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs b/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
index 8f2be75e0f..6b41c1c66c 100644
--- a/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
+++ b/src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs
@@ -1,26 +1,59 @@
-#nullable enable
-using System;
+using System;
+
+#nullable enable
 
 namespace Avalonia.LogicalTree
 {
     /// <summary>
-    /// Event args for <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
+    /// Describes the action that caused a <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
     /// </summary>
-    public class ChildIndexChangedEventArgs : EventArgs
+    public enum ChildIndexChangedAction
     {
-        public static new ChildIndexChangedEventArgs Empty { get; } = new ChildIndexChangedEventArgs();
+        /// <summary>
+        /// The index of a single child changed.
+        /// </summary>
+        ChildIndexChanged,
 
-        private ChildIndexChangedEventArgs()
-        {
-            Index = -1;
-        }
+        /// <summary>
+        /// The index of multiple children changed and all children should be re-evaluated.
+        /// </summary>
+        ChildIndexesReset,
+
+        /// <summary>
+        /// The total number of children changed.
+        /// </summary>
+        TotalCountChanged,
+    }
 
+    /// <summary>
+    /// Event args for <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
+    /// </summary>
+    public class ChildIndexChangedEventArgs : EventArgs
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ChildIndexChangedEventArgs"/> class with
+        /// an action of <see cref="ChildIndexChangedAction.ChildIndexChanged"/>.
+        /// </summary>
+        /// <param name="child">The child whose index was changed.</param>
+        /// <param name="index">The new index of the child.</param>
         public ChildIndexChangedEventArgs(ILogical child, int index)
         {
+            Action = ChildIndexChangedAction.ChildIndexChanged;
             Child = child;
             Index = index;
         }
 
+        private ChildIndexChangedEventArgs(ChildIndexChangedAction action)
+        {
+            Action = action;
+            Index = -1;
+        }
+
+        /// <summary>
+        /// Gets the type of change action that ocurred on the list control.
+        /// </summary>
+        public ChildIndexChangedAction Action { get; }
+
         /// <summary>
         /// Gets the logical child whose index was changed or null if all children should be re-evaluated.
         /// </summary>
@@ -30,5 +63,17 @@ namespace Avalonia.LogicalTree
         /// Gets the new index of <see cref="Child"/> or -1 if all children should be re-evaluated.
         /// </summary>
         public int Index { get; }
+
+        /// <summary>
+        /// Gets an instance of the <see cref="ChildIndexChangedEventArgs"/> with an action of
+        /// <see cref="ChildIndexChangedAction.ChildIndexesReset"/>.
+        /// </summary>
+        public static ChildIndexChangedEventArgs ChildIndexesReset { get; } = new(ChildIndexChangedAction.ChildIndexesReset);
+
+        /// <summary>
+        /// Gets an instance of the <see cref="ChildIndexChangedEventArgs"/> with an action of
+        /// <see cref="ChildIndexChangedAction.TotalCountChanged"/>.
+        /// </summary>
+        public static ChildIndexChangedEventArgs TotalCountChanged { get; } = new(ChildIndexChangedAction.TotalCountChanged);
     }
 }
diff --git a/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs b/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
index a5e191d63e..186c9527f2 100644
--- a/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
+++ b/src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs
@@ -28,10 +28,5 @@ namespace Avalonia.LogicalTree
         /// Notifies subscriber when a child's index was changed.
         /// </summary>
         event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
-
-        /// <summary>
-        /// Notifies subscriber when the total child count changes.
-        /// </summary>
-        event EventHandler<EventArgs>? TotalCountChanged;
     }
 }
diff --git a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
index e0fa056765..8bdcec2e53 100644
--- a/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
+++ b/src/Avalonia.Base/Styling/Activators/NthChildActivator.cs
@@ -37,7 +37,6 @@ namespace Avalonia.Styling.Activators
         protected override void Initialize()
         {
             _provider.ChildIndexChanged += ChildIndexChanged;
-            _provider.TotalCountChanged += TotalCountChanged;
         }
 
         protected override void Deinitialize()
@@ -48,9 +47,11 @@ namespace Avalonia.Styling.Activators
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // Run matching again if:
-            // 1. e.Child is null, when all children indices were changed.
-            // 2. Subscribed child index was changed.
-            if (e.Child is null || e.Child == _control)
+            // 1. Subscribed child index was changed
+            // 2. Child indexes were reset
+            // 3. We're a reversed (nth-last-child) selector and total count has changed
+            if ((e.Child == _control || e.Action == ChildIndexChangedAction.ChildIndexesReset) ||
+                (_reversed && e.Action == ChildIndexChangedAction.TotalCountChanged))
             {
                 // We're using the _index field to pass the index of the child to EvaluateIsActive
                 // *only* when the active state is re-evaluated via this event handler. The docs
diff --git a/src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs b/src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs
index 06a77f0894..f5db7c0855 100644
--- a/src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs
+++ b/src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs
@@ -336,7 +336,7 @@ namespace Avalonia.Controls.Primitives
 
         internal void InvalidateChildIndex()
         {
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
 
         private bool ShouldDisplayCell(DataGridColumn column, double frozenLeftEdge, double scrollingLeftEdge)
diff --git a/src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs b/src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs
index f9b84793c6..fcf72385b2 100644
--- a/src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs
+++ b/src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs
@@ -423,7 +423,7 @@ namespace Avalonia.Controls.Primitives
 
         internal void InvalidateChildIndex()
         {
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
     }
 }
diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 8c3c65a6a7..9483f98881 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -101,7 +101,6 @@ namespace Avalonia.Controls
         private int _itemCount;
         private ItemContainerGenerator? _itemContainerGenerator;
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
-        private EventHandler<EventArgs>? _totalCountChanged;
         private IDataTemplate? _displayMemberItemTemplate;
         private ScrollViewer? _scrollViewer;
         private ItemsPresenter? _itemsPresenter;
@@ -218,12 +217,6 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
-        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
-        {
-            add => _totalCountChanged += value;
-            remove => _totalCountChanged -= value;
-        }
-
         /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> HorizontalSnapPointsChanged
         {
@@ -500,7 +493,7 @@ namespace Avalonia.Controls
             else if (change.Property == ItemCountProperty)
             {
                 UpdatePseudoClasses(change.GetNewValue<int>());
-                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+                _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.TotalCountChanged);
             }
             else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
             {
@@ -585,7 +578,7 @@ namespace Avalonia.Controls
         internal void RegisterItemsPresenter(ItemsPresenter presenter)
         {
             Presenter = presenter;
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
 
         internal void PrepareItemContainer(Control container, object? item, int index)
diff --git a/src/Avalonia.Controls/Panel.cs b/src/Avalonia.Controls/Panel.cs
index 3006efb7de..10391afca2 100644
--- a/src/Avalonia.Controls/Panel.cs
+++ b/src/Avalonia.Controls/Panel.cs
@@ -35,7 +35,6 @@ namespace Avalonia.Controls
         }
 
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
-        private EventHandler<EventArgs>? _totalCountChanged;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
@@ -67,12 +66,6 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
-        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
-        {
-            add => _totalCountChanged += value;
-            remove => _totalCountChanged -= value;
-        }
-
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -161,7 +154,7 @@ namespace Avalonia.Controls
                     throw new NotSupportedException();
             }
 
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
             InvalidateMeasureOnChildrenChanged();
         }
 
@@ -173,7 +166,7 @@ namespace Avalonia.Controls
         private void ChildrenPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == nameof(Children.Count) || e.PropertyName is null)
-                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+                _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.TotalCountChanged);
         }
 
         private static void AffectsParentArrangeInvalidate<TPanel>(AvaloniaPropertyChangedEventArgs e)

From 87dc9bb1b4d37c9c8435c2b0e5bc48606c502d85 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 22 Feb 2023 18:23:16 +0100
Subject: [PATCH 9/9] Only subscribe to event when necessary.

---
 src/Avalonia.Controls/Panel.cs | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Controls/Panel.cs b/src/Avalonia.Controls/Panel.cs
index 10391afca2..fa18ee468c 100644
--- a/src/Avalonia.Controls/Panel.cs
+++ b/src/Avalonia.Controls/Panel.cs
@@ -42,7 +42,6 @@ namespace Avalonia.Controls
         public Panel()
         {
             Children.CollectionChanged += ChildrenChanged;
-            Children.PropertyChanged += ChildrenPropertyChanged;
         }
 
         /// <summary>
@@ -62,8 +61,19 @@ namespace Avalonia.Controls
 
         event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
-            add => _childIndexChanged += value;
-            remove => _childIndexChanged -= value;
+            add
+            {
+                if (_childIndexChanged is null)
+                    Children.PropertyChanged += ChildrenPropertyChanged;
+                _childIndexChanged += value;
+            }
+
+            remove
+            {
+                _childIndexChanged -= value;
+                if (_childIndexChanged is null)
+                    Children.PropertyChanged -= ChildrenPropertyChanged;
+            }
         }
 
         /// <summary>