Copy/paste in Firefox from side-by-side diff loses last line

Summary

When copy/pasting from a side-by-side diff in a merge request, the last line is not copied.

Steps to reproduce

  1. Using Firefox, view a merge request diff in side-by-side view, eg: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3812/diffs?view=parallel
  2. Copy/paste two adjacent additions/removals, eg. The added lines 13/34 in app/assets/javascripts/init_notes.js in the above merge request.
  3. Paste the text into a text editor - the last line will be missing

Example Project

Any merge request, eg. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3812/diffs?view=parallel

What is the current bug behavior?

A copied line is missing.

What is the expected correct behavior?

All copied text should be visible.

Relevant logs and/or screenshots

Sample selected text which causes this issue: image

Output of checks

This bug happens on GitLab.com.

Additional info

This happens in Firefox 57.0.2 (64-bit) on Windows 10. It does not happen with Chrome 63. If the diff is in inline mode, this issue does not occur.