When using the username autocomplete feature in the editor and using enter to select the autocomplete, in XF2.2 this would inject a space character. in XF2.3 this injects a new line.
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.3.0 Beta 3).
Change log:
Prevent new line insertion when hitting enter or tab when selecting an autocomplete result.