Inspired by a past-perfect vs. present-perfect online test:
They were angry because they had waited for too long.
They were angry because they had been waiting for too long.
Are both options grammatical? If yes, what is the difference in meaning?
The second options seems to be more widespread, according to a quick Google search.
Is it because with had waited, the process of waiting might have ended before them becoming angry, and so there is a gap between their "waiting" stopped and their "angriness" started?