When I copy an unpublished comment on this very page here (Talk:Content_translation) that spans multiple rows inside the editing box out of the editing box via my system's clipboard and copy it back into a new box (reload the page, start a new thread), I end up with those visible line break signs inserted: ↵ ( <_| )
Topic on Talk:Content translation
Thanks for reporting this Laotinant. The discussion system we are using in this page is Flow. Flow is a different project also in development by the Wikimedia Foundation, and you can report the specific issues on their own discussion page. From what I understand this may be related to the way copy and paste is dealt with Flow's rich text editing surface, but people with a deeper knowledge of Flow's internals can probably better identify (or even solve) the issue.