Expose macOS ScrollViewer scroll bars to accessibility (#21596)

ScrollViewerAutomationPeer surfaces the template scroll bar peers, which the native peer returns as AXHorizontalScrollBar and AXVerticalScrollBar.

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

@@ -358,6 +358,20 @@
     return [AvnAccessibilityElement acquire:rootPeer];
 }
 
+- (id)accessibilityHorizontalScrollBar
+{
+    if (_peer == nullptr)
+        return nil;
+    return [AvnAccessibilityElement acquire:_peer->ScrollProvider_GetHorizontalScrollBar()];
+}
+
+- (id)accessibilityVerticalScrollBar
+{
+    if (_peer == nullptr)
+        return nil;
+    return [AvnAccessibilityElement acquire:_peer->ScrollProvider_GetVerticalScrollBar()];
+}
+
 - (BOOL)isAccessibilityExpanded
 {
     if (!_peer->IsExpandCollapseProvider())

src/Avalonia.Controls/Automation/Peers/ScrollViewerAutomationPeer.cs

@@ -14,6 +14,12 @@ namespace Avalonia.Automation.Peers
 
         public new ScrollViewer Owner => (ScrollViewer)base.Owner;
 
+        internal AutomationPeer? GetHorizontalScrollBarPeer()
+            => Owner.HorizontalScrollBar is { } scrollBar ? GetOrCreate(scrollBar) : null;
+
+        internal AutomationPeer? GetVerticalScrollBarPeer()
+            => Owner.VerticalScrollBar is { } scrollBar ? GetOrCreate(scrollBar) : null;
+
         public bool HorizontallyScrollable
         {
             get => MathUtilities.GreaterThan(Owner.Extent.Width, Owner.Viewport.Width);

src/Avalonia.Controls/ScrollViewer.cs

@@ -169,6 +169,18 @@ namespace Avalonia.Controls
         private Size _smallChange = new Size(DefaultSmallChange, DefaultSmallChange);
         private bool _isExpanded;
         private IDisposable? _scrollBarExpandSubscription;
+        private ScrollBar? _horizontalScrollBar;
+        private ScrollBar? _verticalScrollBar;
+
+        /// <summary>
+        /// Gets the horizontal scroll bar from the applied template, if any.
+        /// </summary>
+        internal ScrollBar? HorizontalScrollBar => _horizontalScrollBar;
+
+        /// <summary>
+        /// Gets the vertical scroll bar from the applied template, if any.
+        /// </summary>
+        internal ScrollBar? VerticalScrollBar => _verticalScrollBar;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ScrollViewer"/> class.
@@ -834,6 +846,9 @@ namespace Avalonia.Controls
             var horizontalScrollBar = e.NameScope.Find<ScrollBar>("PART_HorizontalScrollBar");
             var verticalScrollBar = e.NameScope.Find<ScrollBar>("PART_VerticalScrollBar");
 
+            _horizontalScrollBar = horizontalScrollBar;
+            _verticalScrollBar = verticalScrollBar;
+
             var horizontalExpanded = GetExpandedObservable(horizontalScrollBar);
             var verticalExpanded = GetExpandedObservable(verticalScrollBar);
 

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -182,6 +182,12 @@ namespace Avalonia.Native
 
         public int IsSelectionItemProvider() => IsProvider<ISelectionItemProvider>();
         public int SelectionItemProvider_IsSelected() => SelectionItemProvider.IsSelected.AsComBool();
+
+        public IAvnAutomationPeer? ScrollProvider_GetHorizontalScrollBar()
+            => _inner is ScrollViewerAutomationPeer scrollViewer ? Wrap(scrollViewer.GetHorizontalScrollBarPeer()) : null;
+
+        public IAvnAutomationPeer? ScrollProvider_GetVerticalScrollBar()
+            => _inner is ScrollViewerAutomationPeer scrollViewer ? Wrap(scrollViewer.GetVerticalScrollBarPeer()) : null;
         
         public int IsToggleProvider() => IsProvider<IToggleProvider>();
         public int ToggleProvider_GetToggleState() => (int)ToggleProvider.ToggleState;

src/Avalonia.Native/avn.idl

@@ -1317,7 +1317,10 @@ interface IAvnAutomationPeer : IUnknown
      
      bool IsSelectionItemProvider();
      bool SelectionItemProvider_IsSelected();
-     
+
+     IAvnAutomationPeer* ScrollProvider_GetHorizontalScrollBar();
+     IAvnAutomationPeer* ScrollProvider_GetVerticalScrollBar();
+
      bool IsToggleProvider();
      int ToggleProvider_GetToggleState();
      void ToggleProvider_Toggle();