WebIDE no longer highlights MR changes

Summary

As of 12.7.2, the red/green diffs for MRs in the WebIDE are gone.

⚠ The big is Firefox on Debian specific and seems to be working fine in other environments.

Steps to reproduce

• Create a MR that makes any change at all to a text file.
• From the MR header, click on "Open in WebIDE"
• You end up in the 2-pane "Review" view, with "before" on the left and "after" on the right
• It used to be the case that the proposed changes would show up highlighted in red/green. This is no longer the case.

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behavior, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug is fixed in a more recent version)

What is the current bug behavior?

No highlighting.

What is the expected correct behavior?

Highlighting appears.

Relevant logs and/or screenshots

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Debian testing
Current User:	git
Using RVM:	no
Ruby Version:	2.6.5p114
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.7
Git Version:	2.24.1
Sidekiq Version:5.2.7
Go Version:	go1.13.6 linux/amd64
GitLab information
Version:	12.7.2
Revision:	2d4cda9da36
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	12.1
URL:		https://gitlab.tiker.net
HTTP Clone URL:	https://gitlab.tiker.net/some-group/some-project.git
SSH Clone URL:	git@gitlab.tiker.net:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:
GitLab Shell
Version:	11.0.0
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 11.0.0 ? ... OK (11.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Checking Reply by email ...
IMAP server credentials are correct? ... yes
Init.d configured correctly? ... skipped
MailRoom running? ... skipped
Checking Reply by email ... Finished
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
2/2 ... yes
64/3 ... yes
64/4 ... yes
64/5 ... yes
64/6 ... yes
64/7 ... yes
2/8 ... yes
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64/10 ... yes
47/11 ... yes
14/12 ... yes
2/13 ... yes
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2/15 ... yes
19/16 ... yes
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65/21 ... yes
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63/38 ... yes
32/39 ... yes
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2/57 ... yes
32/60 ... yes
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29/68 ... yes
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33/75 ... yes
2/76 ... yes
63/78 ... yes
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37/85 ... yes
29/86 ... yes
2/87 ... yes
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2/96 ... yes
47/102 ... yes
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24/109 ... yes
2/110 ... yes
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65/112 ... yes
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45/116 ... yes
2/117 ... yes
63/118 ... yes
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64/127 ... yes
68/128 ... yes
2/129 ... yes
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24/134 ... yes
45/136 ... yes
68/137 ... yes
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13/156 ... yes
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2/303 ... yes
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107/311 ... yes
64/312 ... yes
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129/314 ... yes
129/315 ... yes
61/316 ... yes
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68/322 ... yes
2/323 ... yes
107/324 ... yes
68/325 ... yes
132/326 ... yes
45/327 ... yes
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2/378 ... yes
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68/384 ... yes
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121/410 ... yes
128/412 ... yes
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128/415 ... yes
2/416 ... yes
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64/418 ... yes
134/419 ... yes
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47/506 ... yes
121/507 ... yes
128/509 ... yes
65/510 ... yes
63/511 ... yes
45/512 ... yes
2/513 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.5)
Git version >= 2.22.0 ? ... yes (2.24.1)
Git user has default SSH configuration? ... yes
Active users: ... 94
Is authorized keys file accessible? ... yes
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished