Autocompletion dropdown is misaligned for multi-line text in Firefox

Steps to reproduce

  • use Firefox
  • write a multi-line comment
  • add a user mention

What is the current bug behavior?

auto-completion dropdown is wrongly positioned:

Screen_Shot_2018-09-27_at_11.55.22

What is the expected correct behavior?

auto-completion dropdown is correctly positioned:

Screen_Shot_2018-09-27_at_12.05.15