Unable to add comment on large diff

Summary

I'm trying to add comments on my own MR on one the diffs, but when I click Comment, the comment is not added. No emails are sent out and nothing appears in the Discussion section either. The diff in question has a lot of changes - so much so that it says "Cannot display changes" unless I enable "Hide whitespace". If that makes any difference, I'm commenting on the changed lines. Commenting on other, smaller, diffs in the same MR works.

Steps to reproduce

1. Submit MR with a large diff containing many whitespace changes.
2. Make sure diff cannot be displayed with whitespace shown.
3. Hide whitespace.
4. Make sure diff is now displayed.
5. Type in a comment on one of the changed lines.
6. Click "Comment".

Adding a comment on an unchanged line of a diff often causes the same behavior (doesn't matter if the diff is big or small).

I have Firefox 55.0.2 64-bit, Ubuntu 16.04.

What is the current bug behavior?

Comment is not added after "Comment" is clicked.

What is the expected correct behavior?

Comment is successfully added.

Relevant logs and/or screenshots

None

Output of checks

This bug happens on GitLab.com.