Taking screen and window shots is easy (at least in gnome based ununtu). But the methods I have tried to take that shot to show a drop down menu close the menu before the shot is taken.
Specifically, using the Print Screen key, ALT + Print Screen, and using the Take Screen Shot Entry on the System menu drop the pulldown menu. Note that the dropdown menu stays up even on switching focus to another window and even while clicking on the system menu.
Searching the web seems to show about a dozen utilities for taking screen shots in Linux but none seem to mention anything about this. I found one method that looks like it might work for a window opened for an application on a remote system but that is not practical here.