How to capture a drop-down menu?

Run the Screenshot program (gnome-screenshot, or search for Screenshot in the Dash). Set a timeout (or delay):

And then get the menu and wait for the timeout to expire.

This only works with a whole-screen screenshot. So you'll have to manually crop later:

This is really simple using shutter and the best thing about using shutter is that you don't get to crop your menu later. It just captures the menu specified.

1. Install and launch shutter

   sudo apt-get install shutter
   

2. Using the shutter window, click on the 'menu' icon. See screenshot below:

   

3. You will then be prompted to choose and click on which menu you want captured. Let's take for example Firefox - my browser. See screenshot below:

   

Install the imagemagick package, then issue the following command on a terminal:

$ sleep 10; import -window root /tmp/screenshot.png

In the next 10 seconds change your active window, open the dropdown menu and wait. You will likely need to crop your screenshot afterwards.