Copying from inline diff only copies first line in FF

Summary

Copying lines from a diff should copy all the lines selected, not just the first line.

Steps to reproduce

On Firefox:

  1. Visit a diff (inline) (e.g. on MR page)
  2. Select a number of lines
  3. Copy

What is the current bug behavior?

It copies only the first line to clipboard.

What is the expected correct behavior?

Copy all the selected lines to clipboard.

Relevant logs and/or screenshots

image

Output of checks

This bug happens on GitLab.com

Related Issues/MRs

gitlab-org/gitlab-ce!8597