Steps to reproduce:
In a topic, type “Hi @username,<enter>”
Text box contains “Hi @username,” and a newline. Also, the username autocompletion box should go away after typing the comma.
Text box contains “Hi @username” and no newline. In other words, because the autocomplete stayed open after typing the comma, the <enter> was interpreted as selecting the username from the autocomplete. This wipes out the comma, and means that no newline is inserted.
This is on hosted Discourse, so presumably the latest version. https://community.letsencrypt.org/.
Thanks very much!