Whenever I am responding by using "add a comment" I always begin with @username. Here are what I observed and am curious for their explanations:
(1) Sometimes the system prompts the complete username after I just type in the first few letters, but sometimes the system does nothing.
(2) Sometimes the @username disappears after I post it, presumably indicating that it has been acknowledged by the system, but in some other times the username stays.
Thank you for your explanation.