Menus and dialogs do not work when using VNC

Previously XCB crashed when no monitor was connected to a Raspberry Pi board but only a VNC connection. This bug has been fixed here: QTBUG-104443.

But now menus and (some) dialogs do not work anymore during a VNC session.

Steps to reproduce:

1. Make sure that no monitor is connected to the Raspberry Pi.
2. Power on the Pi and connect via VNC.
3. Run the attached example and click on the "File" menu. Nothing happens.
4. Connect a monitor to the Pi and restart it.
5. Start the attached example and click on the "File" menu again. The menu opens.

The same happens when you try to open a QDialog (e.g. QMessageBox) from the main window.