Seems to have been caused by https://gerrit.wikimedia.org/r/c/mediawiki/core/+/939783 (T341244)

Looks like the MobileFrontend MakeSectionsTransform is no longer working correctly with presumably-changed Parsoid markup -- the linked beta page has the entire article content wrapped in <section class="mf-section-0" id="mf-section-0">.

I thought MFE didn't use Parsoid HTML?

Looks like this query selector in MFE is the problem:

		$containers = $xpath->query( 'body/div[@class="mw-parser-output"][1]' );

After the change, that query selector no longer applies. Patch incoming.

I guess that patch changed the base Parser output as well, so it really could be either.

It looks like all the patch should have done is chang the wrapper from <div class="mw-parser-output"> to <div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr">. So presumably it's some selector somewhere that expects these classes to be on different elements, or something.

EDIT: sorry, didn't see that reply just before I posted.

Quick explanation of the issue:

1. [@class="mw-parser-output"] is checking for an exact-match of the class attribute.
2. The parser patch added more classes to the mw-parser-output div.
3. class="mw-content-ltr mw-parser-output" is no-longer an exact match.

This bug is the XPath-equivalent of the CSS-selector [rel="link"] issue, that we've slowly been fixing by replacing those with [rel~="link"] when we notice them. Sadly, XPath doesn't have an elegant query quite like that, but there's workarounds.

Change 972058 had a related patch set uploaded (by DLynch; author: DLynch):

[mediawiki/extensions/MobileFrontend@master] Fix xpath selector in MakeSectionsTransform for multiple classes

https://gerrit.wikimedia.org/r/972058

Change 972058 merged by jenkins-bot:

[mediawiki/extensions/MobileFrontend@master] Fix xpath selector in MakeSectionsTransform for multiple classes

https://gerrit.wikimedia.org/r/972058

ssastry did a code search and noticed that there's one non-WMF skin that was going to be hit by the same xpath issue, so I made a pull request on it on github.

Thanks everyone for the quick and thoughtful response here!

Test Result - Prod

Status: ✅ PASS
Environment: enwiki
OS: macOS Sonoma
Browser: Chrome
Device: MBA
Emulated Device:NA

Test Artifact(s):

QA Steps

✅ AC1: Sections should be collapsible on mobile pages.