As soon as a user types an @ symbol the below mention popup appears and the cursor is redirected into the mention popup use field.
If a user simply wants to type " @ " they are currently required to grab the mouse, click cancel and then resume typing.
The same essentially happens if a user just wants to type "@github.com" for example.
An initial step in a good direction would be to solve the issue with using a space after an @ symbol, which could just close the popup and continue typing.
A further step could be to stop the mention popup from taking focus away from the main text field and instead allow the user to continue typing there while continuing to suggest users to mention (in the same style as phabricator)