Don't modify logical tree in ItemsSource mode.

src/Avalonia.Controls/ItemsControl.cs

@@ -573,15 +573,18 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         private protected virtual void OnItemsViewCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            switch (e.Action)
+            if (!_items.IsReadOnly)
             {
-                case NotifyCollectionChangedAction.Add:
-                    AddControlItemsToLogicalChildren(e.NewItems);
-                    break;
+                switch (e.Action)
+                {
+                    case NotifyCollectionChangedAction.Add:
+                        AddControlItemsToLogicalChildren(e.NewItems);
+                        break;
 
-                case NotifyCollectionChangedAction.Remove:
-                    RemoveControlItemsFromLogicalChildren(e.OldItems);
-                    break;
+                    case NotifyCollectionChangedAction.Remove:
+                        RemoveControlItemsFromLogicalChildren(e.OldItems);
+                        break;
+                }
             }
 
             ItemCount = ItemsView.Count;

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -249,6 +249,37 @@ namespace Avalonia.Controls.UnitTests
             Assert.Null(((ILogical)child).LogicalParent);
         }
 
+        [Fact]
+        public void Assigning_ItemsSource_Should_Not_Fire_LogicalChildren_CollectionChanged_Before_ApplyTemplate()
+        {
+            var target = new ItemsControl();
+            var child = new Control();
+            var called = false;
+
+            ((ILogical)target).LogicalChildren.CollectionChanged += (s, e) => called = true;
+
+            var list = new AvaloniaList<Control>(new[] { child });
+            target.ItemsSource = list;
+
+            Assert.False(called);
+        }
+
+        [Fact]
+        public void Changing_ItemsSource_Should_Not_Fire_LogicalChildren_CollectionChanged_Before_ApplyTemplate()
+        {
+            var target = new ItemsControl();
+            var child = new Control();
+            var called = false;
+
+            ((ILogical)target).LogicalChildren.CollectionChanged += (s, e) => called = true;
+
+            var list = new AvaloniaList<Control>();
+            target.ItemsSource = list;
+            list.Add(child);
+
+            Assert.False(called);
+        }
+
         [Fact]
         public void Clearing_Items_Should_Clear_Child_Controls_Parent()
         {