Mention dropdown does not show when you type @ at the beginning of second line

Summary

Dropdown with a list of users / groups that you can mention is no longer showing when you type @ at the beginning of the second line

Steps to reproduce

1. Go to a comment box
2. Type @ at the beginning of the first line
3. Dropdown shows up
4. Remove @ and type test in the first line
5. Hit enter
6. Type @ at the beginning of the second line
7. Dropdown doesn't show up
8. Remove @ and type a<SPACE> in the second line
9. Type @
10. Dropdown shows up

/cc @smcgivern