I seem to be running into a problem when attempting to reply to a comment from a user whose user name has a space in it. When I use the @ symbol and begin typing the username, the name will appear is a small popup which I can then click and it will be entered into the comment.
However if the name has spaces then the username in the popup as well as the user name automatically inserted into the comment does not have spaces.
The result is that the username that is automatically inserted into the comment is incorrect.
For instance in my response to a comment to an answer I provided, https://raspberrypi.stackexchange.com/a/77672/73672 , it appears that I need to add a space to the name from the popup and when I do the user is notified via their global mailbox.