Taking a screenshot in GNOME desktop during a pulldown menu is open. How to?

[Joachim Backes]

Hi all,
having a problem: I need to take a screenshot on the gnome desktop (no 
wayland!)with alt+print or shift+print keys, during some pulldown menu 
of some window is opened,  but nothing happens. How can I achive this? 
The pulldown menu should be included in the screenshot. The screenshot 
is performed flawlessly if the pulldown menu is closed.

It seems that this is a X11 problem.

All comments are welcome.

Kind regards

Joachim Backes