Fix missing root automation peer in x11/atspi (#20775)

* Fix missing root automation peer in x11/atspi

* Root nodes are special case; fix GetIndexInParentAsync to handle root nodes since their parents == null.

src/Avalonia.FreeDesktop.AtSpi/Handlers/AtSpiAccessibleHandler.cs

@@ -63,6 +63,22 @@ namespace Avalonia.FreeDesktop.AtSpi.Handlers
 
         public ValueTask<int> GetIndexInParentAsync()
         {
+            // Window nodes are children of the ApplicationAtSpiNode, but their
+            // internal Parent field is null (they are attached with parent: null).
+            // Mirror the Parent property's special case so that backward path
+            // walks (e.g. accerciser's get_index_in_parent) work correctly.
+            if (node is RootAtSpiNode { AppRoot: { } appRoot })
+            {
+                var windows = appRoot.WindowChildren;
+                for (var i = 0; i < windows.Count; i++)
+                {
+                    if (ReferenceEquals(windows[i], node))
+                        return ValueTask.FromResult(i);
+                }
+
+                return ValueTask.FromResult(-1);
+            }
+
             var parent = node.Parent;
             if (parent is null)
                 return ValueTask.FromResult(-1);

src/Avalonia.X11/X11AtSpiAccessibility.cs

@@ -142,8 +142,11 @@ namespace Avalonia.X11
         {
             try
             {
-                if (window.InputRoot.RootElement is Control control)
-                    return ControlAutomationPeer.CreatePeerForElement(control);
+                if (window.InputRoot.FocusRoot is Control control)
+                {
+                    var peer = ControlAutomationPeer.CreatePeerForElement(control);
+                    return peer?.GetAutomationRoot() ?? peer;
+                }
             }
             catch (Exception e)
             {

src/Avalonia.X11/X11Window.cs

@@ -1029,9 +1029,10 @@ namespace Avalonia.X11
             // Remove from AT-SPI tree before closing
             _platform.UntrackWindow(this);
             if (_platform.AtSpiServer is { } atSpiServer
-                && _inputRoot?.RootElement is Control atSpiControl)
+                && _inputRoot?.FocusRoot is Control atSpiControl)
             {
-                var atSpiPeer = atSpiControl.GetAutomationPeer();
+                var atSpiPeer = atSpiControl.GetAutomationPeer()?.GetAutomationRoot()
+                    ?? atSpiControl.GetAutomationPeer();
                 if (atSpiPeer is not null)
                     atSpiServer.RemoveWindow(atSpiPeer);
             }
@@ -1125,11 +1126,12 @@ namespace Avalonia.X11
 
             _platform.TrackWindow(this);
             if (_platform.AtSpiServer is { } server
-                && _inputRoot?.RootElement is Control c)
+                && _inputRoot?.FocusRoot is Control c)
             {
                 var peer = Avalonia.Automation.Peers.ControlAutomationPeer.CreatePeerForElement(c);
-                if (peer is not null)
-                    server.AddWindow(peer);
+                var rootPeer = peer?.GetAutomationRoot() ?? peer;
+                if (rootPeer is not null)
+                    server.AddWindow(rootPeer);
             }
         }