How can I take a screenshot with a popup window in it, like those I see in some replies?

I have set set a gesture for screenshot, but nothing happens, when a popup is open.

Thanks!
A full diagonal swipe from corner to corner should normally do the trick. If it's a multitouch device you can also press two diagonal corners at once.

But if you're talking about the screenshots I posted, those were from the desktop version. What you see is the same, but since you don't have to copy them over from the device it's quicker that way.