Fix unopened toplevel menu selection with down key.

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -149,13 +149,18 @@ namespace Avalonia.Controls.Platform
                 case Key.Up:
                 case Key.Down:
                 {
-                    if (item?.IsTopLevel == true)
+                    if (item?.IsTopLevel == true && item.HasSubMenu)
                     {
-                        if (item.HasSubMenu && !item.IsSubMenuOpen)
+                        if (!item.IsSubMenuOpen)
                         {
                             Open(item, true);
-                            e.Handled = true;
                         }
+                        else
+                        {
+                            item.MoveSelection(NavigationDirection.First, true);
+                        }
+
+                        e.Handled = true;
                     }
                     else
                     {

tests/Avalonia.Controls.UnitTests/Platform/DefaultMenuInteractionHandlerTests.cs

@@ -53,6 +53,19 @@ namespace Avalonia.Controls.UnitTests.Platform
                 Assert.True(e.Handled);
             }
 
+            [Fact]
+            public void Down_Selects_First_Item_Of_Already_Opened_Submenu()
+            {
+                var target = new DefaultMenuInteractionHandler(false);
+                var item = Mock.Of<IMenuItem>(x => x.IsTopLevel == true && x.HasSubMenu == true && x.IsSubMenuOpen);
+                var e = new KeyEventArgs { Key = Key.Down, Source = item };
+
+                target.KeyDown(item, e);
+
+                Mock.Get(item).Verify(x => x.MoveSelection(NavigationDirection.First, true));
+                Assert.True(e.Handled);
+            }
+
             [Fact]
             public void Right_Selects_Next_MenuItem()
             {