Menus become detached from Qt windows

To reproduce:

1. Open some Qt program that has a menu.
2. Click the menu to open it.
3. Move the Qt window to a different position on screen.
4. Click the menu again.

Result: the menu popup appears at the old window position.

Affects git revision 34457549

Does not affect the version of Xfwm in Ubuntu 20.04 (4.14.1 (revision 44809c49))