🚀 go-pugleaf

I have a problem with the Built-in screen shooter on Mac.
The shooter is supposed to take a snapshot of the screen
and stores a picture of the screen on disk, when the keys
Command-Shift-3 are pressed.

It works for me except when I want to take a snapshot 
of the screen while a pull-down menu is selected(without
releasing the mouse button). In that case, the snapshot
is simply a picture as if I didn't make a menu selection.

But I have seen snapshots of screen while a menu is selected
from various software manuals. How it can be done? What is 
the command? I need this because I need to show people how
to use a certain application before they can get on Mac,
with the actual pictures showing what will be happenning
when the application is launched.

Thanks in advance.