close menu if the toplevel menu item is clicked and its already opened.

makes behavior consistent with wpf.

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -336,7 +336,18 @@ namespace Avalonia.Controls.Platform
 
             if (e.MouseButton == MouseButton.Left && item?.HasSubMenu == true)
             {
-                Open(item, false);
+                if (item.IsSubMenuOpen)
+                {
+                    if (item.Parent is Menu)
+                    {
+                        CloseMenu(item);
+                    }
+                }
+                else
+                {
+                    Open(item, false);
+                }
+
                 e.Handled = true;
             }
         }