Fix macOS AXLayoutChanged notifications dropped for template-internal peers (#21595)

* Post macOS AXLayoutChanged on the changed peer's templated parent

The changed peer is usually a template-internal layout container that AppKit will not deliver notifications for; post on its templated parent (the owning control), falling back to the nearest exposed ancestor when there is none.

* Update with comment

native/Avalonia.Native/src/OSX/automation.mm

@@ -516,8 +516,31 @@
     if (_children)
         [changed addObjectsFromArray:_children];
 
+	/*
+	For future reference, upon testing with a sample SwiftUI app:
+
+    1) Containers vanish. VStack/HStack don't appear in the accessibility tree at all, 
+      only real elements (Text, Button, List) do, parented to one root AXHostingView.
+
+    2) Changes post on the nearest real element, not the container. 
+       Toggling a child two VStack/HStack levels deep fired AXLayoutChanged on the AXHostingView 
+       (the nearest real element), never on the hidden containers.
+
+    3) Real controls get their own notification. The List (AXOutline) posted AXRowCountChanged on itself.
+
+	Apple never posts a structural notification on a non-accessibility container.
+    It targets the nearest element that's actually in the tree.
+    That's exactly what the code below does (templated parent, else walk to the nearest exposed ancestor).
+	*/
+
+    id target = [AvnAccessibilityElement acquire:_peer->GetTemplatedParent()];
+    if (target == nil)
+        target = self;
+    while ([target isKindOfClass:[AvnAccessibilityElement class]] && ![(AvnAccessibilityElement*)target isAccessibilityElement])
+        target = [(AvnAccessibilityElement*)target accessibilityParent];
+
     NSAccessibilityPostNotificationWithUserInfo(
-        self,
+        target,
         NSAccessibilityLayoutChangedNotification,
         @{ NSAccessibilityUIElementsKey: [changed allObjects]});
 }

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -6,6 +6,7 @@ using System.Runtime.CompilerServices;
 using Avalonia.Automation;
 using Avalonia.Automation.Peers;
 using Avalonia.Automation.Provider;
+using Avalonia.Controls;
 using Avalonia.Controls.Automation.Peers;
 using Avalonia.Native.Interop;
 
@@ -56,6 +57,10 @@ namespace Avalonia.Native
         public AvnLandmarkType LandmarkType => (AvnLandmarkType?)_inner.GetLandmarkType() ?? AvnLandmarkType.LandmarkNone;
         public int HeadingLevel => _inner.GetHeadingLevel();
         public IAvnAutomationPeer? Parent => Wrap(_inner.GetParent());
+        public IAvnAutomationPeer? TemplatedParent =>
+            _inner is ControlAutomationPeer { Owner.TemplatedParent: Control templatedParent } ? 
+                Wrap(ControlAutomationPeer.CreatePeerForElement(templatedParent))
+                : null;
         public IAvnAutomationPeer? VisualRoot => Wrap(_inner.GetAutomationRoot());
         public AvnLiveSetting LiveSetting => (AvnLiveSetting)_inner.GetLiveSetting();
 

src/Avalonia.Native/avn.idl

@@ -1274,6 +1274,7 @@ interface IAvnAutomationPeer : IUnknown
      IAvnAutomationPeer* GetLabeledBy();
      IAvnString* GetName();
      IAvnAutomationPeer* GetParent();
+     IAvnAutomationPeer* GetTemplatedParent();
      IAvnAutomationPeer* GetVisualRoot();
      bool HasKeyboardFocus();
      bool IsContentElement();