Implement isAccessibilitySelected.

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

@@ -349,6 +349,13 @@ private:
     return YES;
 }
 
+- (BOOL)isAccessibilitySelected
+{
+    if (_peer->IsSelectionItemProvider())
+        return _peer->SelectionItemProvider_IsSelected();
+    return NO;
+}
+
 - (BOOL)isAccessibilitySelectorAllowed:(SEL)selector
 {
     if (selector == @selector(accessibilityPerformShowMenu))

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -126,6 +126,9 @@ namespace Avalonia.Native
         public double RangeValueProvider_GetSmallChange() => ((IRangeValueProvider)_inner).SmallChange;
         public double RangeValueProvider_GetLargeChange() => ((IRangeValueProvider)_inner).LargeChange;
         public void RangeValueProvider_SetValue(double value) => ((IRangeValueProvider)_inner).SetValue(value);
+
+        public int IsSelectionItemProvider() => (_inner is ISelectionItemProvider).AsComBool();
+        public int SelectionItemProvider_IsSelected() => ((ISelectionItemProvider)_inner).IsSelected.AsComBool();
         
         public int IsToggleProvider() => (_inner is IToggleProvider).AsComBool();
         public int ToggleProvider_GetToggleState() => (int)((IToggleProvider)_inner).ToggleState;

src/Avalonia.Native/avn.idl

@@ -856,6 +856,9 @@ interface IAvnAutomationPeer : IUnknown
      double RangeValueProvider_GetLargeChange();
      void RangeValueProvider_SetValue(double value);
      
+     bool IsSelectionItemProvider();
+     bool SelectionItemProvider_IsSelected();
+     
      bool IsToggleProvider();
      int ToggleProvider_GetToggleState();
      void ToggleProvider_Toggle();