Web IDE no longer highlighting changes in MRs

Summary

When viewing a MR in the Web IDE changes are no longer highlighted. Customer noted upon upgrade to 15.3.1 on their self hosted instance and I can replicate on gitlab.com

I've tested this in FF/Chrome/Safari on gitlab.com (Safari/chrome with a clear cache) and the issue presents.

This does not seem to occur on my personal aarch64 15.3.1 instance from quick testing

Steps to reproduce

Head to any MR on a project
Click the Code drop down and click on Open in Web IDE

Example Project

Quick sample MR here

https://gitlab.com/gitlab-gold/simonstreet-test/webidemrdiff/-/merge_requests/1

What is the current bug behavior?

Changes are not highlighted in the Web IDE side by side diff view for MRs

What is the expected correct behavior?

Changes should be highlighted

Relevant logs and/or screenshots

Screenshot from above project showing lack of highlights:

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info


(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)