Added IChildIndexProvider.TotalCountChanged.

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;
     }
 }

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();
+        }
     }
 }

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)
             {

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
         {