ext.uls.interface is never loaded by any module as a dependency from looking at code search.
This suggests https://gerrit.wikimedia.org/g/mediawiki/extensions/UniversalLanguageSelector/+/5a7a6a5fe30e2e64119c134e7dca7d737a0d4fe3/includes/UniversalLanguageSelectorHooks.php#135 is what loads it.

Something looks off with this logic. Both ULSEnableAnon and ULSEnable are true by default, but this function does not consider the preferences setting that requests that the code is never loaded.

@Nikerabbit @santhosh is there a reason this module is always loaded?

Perhaps Vector needs to also check the value of mw.config.get( 'wgULSisCompactLinksEnabled' ) if that behaviour is correct.

Both ULSEnableAnon and ULSEnable are true by default, but this function does not consider the preferences setting that requests that the code is never loaded.

Those two variables should probably be removed altogether.

There is no preference setting for disabling ULS. We expanded the scope of "disable compact language links" to "disable new vector language selector". It does not make sense to expand it further to "disable uls". I think you want to check that variables whether ULS attaches itself to the new language selector, as that is what ULS does: https://gerrit.wikimedia.org/g/mediawiki/extensions/UniversalLanguageSelector/+/01c7a4fb0b0cb788c5266aebe9388612c49ba20c/resources/js/ext.uls.interface.js#526

Change 708164 had a related patch set uploaded (by Jdlrobson; author: Jdlrobson):

[mediawiki/skins/Vector@master] Do not rely on load order for disabling language button fallback

https://gerrit.wikimedia.org/r/708164

@Nikerabbit I think the logic in the above is more reliable, although not perfect. T286932 might add a hook we could use instead but I think we'd still have the race condition problem if the hook fires before Vector's code loads. Do you think the above is a suitable solution? If so, should I put it through my team's process?

I think that your patch a good simple solution for the time being and you can proceed with your processes.

We can discuss the ideal long term solution separately. Doesn't mw.hook remember events so that you receive events fired even before subscription?

Thanks @Nikerabbit for the guidance here!

We can discuss the ideal long term solution separately. Doesn't mw.hook remember events so that you receive events fired even before subscription?

I don't think it does, but I may be wrong. I'd have to look into the code.

Change 708164 merged by jenkins-bot:

[mediawiki/skins/Vector@master] Do not rely on load order for disabling language button fallback

https://gerrit.wikimedia.org/r/708164

This one can't easily be QAed but I'd suggest we at least verify the button works.

Test Result - Beta

Status: ✅ PASS
Environment: beta
OS: macOS Big Sur
Browser: Chrome
Device: MBP
Emulated Device: NA

Test Artifact(s):

QA Steps

Scenario 1
Visit https://en.wikipedia.beta.wmflabs.org/wiki/Selenium_language_test_page incognito window
Click the language button
✅ AC1: Expected: the dialog shows with "search for language"

Scenario 2
Login
Visit https://en.wikipedia.beta.wmflabs.org/wiki/Special:Preferences and disable the box "Use a compact language list, with languages relevant to you." at bottom of appearance section
Visit https://en.wikipedia.beta.wmflabs.org/wiki/Selenium_language_test_page
Click the language button
✅ AC2: Expected: a scrollable list should appear.
I duplicated the language element to get a scroll bar to appear