window: Don't try to move hidden attached windows

When a parent window is moved, attached windows (attached modal dialogs
or popups) is moved with it. This is problematic when such a window
hasn't been shown yet (e.g. a popup that has been configured but not
shown), as it'll mean we try to constrain an empty window. Avoid this
issue by not trying to auto-move empty windows.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/496

src/core/window.c

@@ -3781,6 +3781,9 @@ static gboolean
 maybe_move_attached_window (MetaWindow *window,
                             void       *data)
 {
+  if (window->hidden)
+    return FALSE;
+
   if (meta_window_is_attached_dialog (window) ||
       meta_window_get_placement_rule (window))
     meta_window_reposition (window);