Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parser
Closed, ResolvedPublic2 Estimated Story PointsBUG REPORT
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Authored By

	lwatson
	Feb 3 2026, 8:59 PM

Description

Steps to replicate the issue (include links if applicable):

Navigate to this URL with the Minerva TOC experiment enabled and legacy parser: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:treatment&useparsoid=0
In a new tab navigate to this URL with the Minerva TOC experiment enabled and legacy parser: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:control&useparsoid=0

What happens?:
Notice that the infobox appears before the lead paragraph in treatment BUT not in control. This means this is not a proper A/B test.

Screenshot 2026-02-03 at 1.49.17 PM.png (1×662 px, 444 KB)

Notice that images are NOT lazy loaded and all load on page load

What should have happened instead?:
The lead paragraph appears before the infobox. This is based on MoveLeadParagraphTransform logic in MobileFrontend.

Screenshot 2026-02-03 at 1.49.35 PM.png (1×666 px, 259 KB)

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

Decision path for fix

Out come of spike on T416503 was that we could make a transform hack that seemed to work, but Jon and Brooke agreed the risk of breaking non-experiment code was greater than we were comfortable with and we plan to force the control group to use parsoid for consistency between the two parts of the experiment, without any effect on non-experiment renderings.

QA notes

(once live on enwiki, or make a copy on beta)
confirming the lead paragraph works when not overridden:
- treatment with no override should show paragraph above the infobox: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:treatment&useskin=minerva
- control with no override should show paragraph above the infobox: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:control&useskin=minerva
confirming that the URL override still takes priority:
- treatment with useparsoid=0 should show *paragraph above the infobox* because user opted into the bug :D : https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:treatment&useskin=minerva&useparsoid=0
- control with useparsoid=0 should show infobox above the paragraph: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:control&useskin=minerva&useparsoid=0
- treatment with useparsoid=1 should show paragraph above the infobox: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:treatment&useskin=minerva&useparsoid=1
- control with useparsoid=1 should show paragraph above the inofbox: https://en.wikipedia.org/wiki/Alfalfa?mpo=minerva-toc-button:control&useskin=minerva&useparsoid=1

Details

Related Changes in Gerrit:

Subject	Repo	Branch	Lines +/-
Override ParserMigration state for Minerva TOC treatment and control	mediawiki/extensions/ReaderExperiments	master	+62 -28
Drop MobileFrontend from ReaderExperiments deps	integration/config	master	+0 -2
Tweak CI deps for ReaderExperiments	integration/config	master	+4 -2
Update CI dependencies for ReaderExperiments	integration/config	master	+12 -0
ShouldUseParsoidHook interface for overriding ParserMigration state	mediawiki/extensions/ParserMigration	master	+83 -1
Fix for lead para transform without legacy section collapsing	mediawiki/extensions/MobileFrontend	master	+89 -0

Customize query in gerrit

Related Objects

Mentioned In: T417818: (CI) MobileFrontend test failures when used as a dep for ReaderExperiments
T416503: [spike] Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parser in treatment but NOT in control
T416244: [Minerva TOC] floating action button bugs
Mentioned Here: T416503: [spike] Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parser in treatment but NOT in control

Event Timeline

lwatson created this task.Feb 3 2026, 8:59 PM

Restricted Application added a subscriber: Aklapper. · View Herald TranscriptFeb 3 2026, 8:59 PM

lwatson assigned this task to Jdlrobson.Feb 3 2026, 9:04 PM

lwatson moved this task from Incoming/Inbox to Backlog on the Reader Growth Team board.

lwatson mentioned this in T416244: [Minerva TOC] floating action button bugs.Feb 3 2026, 9:08 PM

Jdlrobson-WMF claimed this task.Feb 3 2026, 10:37 PM

Jdlrobson-WMF added a subscriber: Jdlrobson.

It looks like the lead paragraph transform has a hidden dependency on section collapsing. This is by design as being able to identify the lead section is what it uses to work out what the lead paragraph is.

So you have several options (assuming you need to support BOTH legacy and Parsoid)

Implement a fallback in `MoveLeadParagraphTransform::apply and LazyImageTransform that works without sections. This seems like a lot of effort so wouldn't recommend.

	public function apply( Element $node ) {
		$section = DOMCompat::querySelector( $node, 'section' );
		if ( $section ) {
			$this->moveFirstParagraphBeforeInfobox( $section, $section->ownerDocument );
		}
	} else {
		// implement fallback here.
        }

Revisit the use of $wgMFNamespacesWithoutCollapsibleSections

This approach can't be used if you want to retain the lead paragraph transform. You'd need to explore an alternative that keeps the sections but disables the JavaScript behaviour.

It's probably medium effort but your best option if you need to do this.

Ship with the lead paragraph/lazy images transform removed. Worry about this when you scale.

Given the experiment introduces the bug in both test groups it's probably okay.

Jdlrobson-WMF renamed this task from Lead paragraph should be before infobox on mobile to Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parser.Feb 4 2026, 12:12 AM

Jdlrobson-WMF updated the task description. (Show Details)

Jdlrobson-WMF moved this task from Backlog to Needs Refinement on the Reader Growth Team board.Feb 4 2026, 5:41 PM

Jdlrobson-WMF triaged this task as High priority.Feb 4 2026, 5:50 PM

Jdlrobson-WMF moved this task from Needs Refinement to Sprint 2 (Feb 3 - 16) Q3 25/26 on the Reader Growth Team board.

Jdlrobson-WMF edited projects, added Reader Growth Team (Sprint 2 (Feb 3 - 16) Q3 25/26); removed Reader Growth Team.

Stalled on T416405: Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parser

Change #1237347 had a related patch set uploaded (by Bvibber; author: Bvibber):

[mediawiki/extensions/MobileFrontend@master] Proposed fix for lead para transform when section collapsing is disabled

https://gerrit.wikimedia.org/r/1237347

gerritbot added a project: Patch-For-Review.Feb 5 2026, 10:09 PM

Spike is looking good. If no objection moving this into code review for the patch.

bvibber moved this task from Code Review to Doing on the Reader Growth Team (Sprint 2 (Feb 3 - 16) Q3 25/26) board.Feb 6 2026, 11:44 PM

Change #1237347 abandoned by Bvibber:

[mediawiki/extensions/MobileFrontend@master] Fix for lead para transform without legacy section collapsing

Reason:

abandoning proposed patch as still too risky; we'll go with switching the control group to parsoid, which is a known quantity

https://gerrit.wikimedia.org/r/1237347

bvibber updated the task description. (Show Details)Feb 6 2026, 11:50 PM

bvibber set the point value for this task to 2.

Maintenance_bot removed a project: Patch-For-Review.Feb 7 2026, 12:31 AM

HSwan-WMF moved this task from Sprint 2 (Feb 3 - 16) Q3 25/26 to Ready on the Reader Growth Team board.Feb 9 2026, 5:18 PM

HSwan-WMF edited projects, added Reader Growth Team; removed Reader Growth Team (Sprint 2 (Feb 3 - 16) Q3 25/26).

Jdlrobson-WMF moved this task from Untriaged to MobileFormatter on the MobileFrontend board.Tue, Feb 10, 7:19 PM

Jdlrobson-WMF edited projects, added MobileFrontend (MobileFormatter); removed MobileFrontend.

Change #1239410 had a related patch set uploaded (by Bvibber; author: Bvibber):

[mediawiki/extensions/ParserMigration@master] ShouldUseParsoidHook interface for overriding ParserMigration state

https://gerrit.wikimedia.org/r/1239410

Change #1239411 had a related patch set uploaded (by Bvibber; author: Bvibber):

[mediawiki/extensions/ReaderExperiments@master] Override ParserMigration state for Minerva TOC treatment and control

https://gerrit.wikimedia.org/r/1239411

Change #1239410 merged by jenkins-bot:

[mediawiki/extensions/ParserMigration@master] ShouldUseParsoidHook interface for overriding ParserMigration state

https://gerrit.wikimedia.org/r/1239410

ReleaseTaggerBot added a project: MW-1.46-notes (1.46.0-wmf.16; 2026-02-17).Tue, Feb 17, 12:00 AM

bvibber edited projects, added Reader Growth Team (Sprint 2 (Feb 3 - 16) Q3 25/26); removed Reader Growth Team.Tue, Feb 17, 5:04 PM

SherryYang-WMF moved this task from Sprint 2 (Feb 3 - 16) Q3 25/26 to Sprint 3 (Feb 17 - Mar 2) Q3 25/26 on the Reader Growth Team board.Tue, Feb 17, 5:41 PM

SherryYang-WMF edited projects, added Reader Growth Team (Sprint 3 (Feb 17 - Mar 2) Q3 25/26); removed Reader Growth Team (Sprint 2 (Feb 3 - 16) Q3 25/26).

egardner moved this task from Committed to Code Review on the Reader Growth Team (Sprint 3 (Feb 17 - Mar 2) Q3 25/26) board.Tue, Feb 17, 6:33 PM

Ok moving forward with alternate patch based on the spike results:

ParserMigration side merged (thanks @cscott!)
ReaderExperiments side about to do...

bvibber updated the task description. (Show Details)Tue, Feb 17, 10:06 PM

Change #1240078 had a related patch set uploaded (by Bvibber; author: Bvibber):

[integration/config@master] Update CI dependencies for ReaderExperiments

https://gerrit.wikimedia.org/r/1240078

Change #1240078 merged by jenkins-bot:

[integration/config@master] Update CI dependencies for ReaderExperiments

https://gerrit.wikimedia.org/r/1240078

Change #1240081 had a related patch set uploaded (by Bvibber; author: Bvibber):

[integration/config@master] Tweak CI deps for ReaderExperiments

https://gerrit.wikimedia.org/r/1240081

Change #1240081 merged by jenkins-bot:

[integration/config@master] Tweak CI deps for ReaderExperiments

https://gerrit.wikimedia.org/r/1240081

Test wiki created on Patch demo by LWatson-WMF using patch(es) linked to this task:
https://b0a5fc90a5.catalyst.wmcloud.org/wiki/Main Page

Change #1240360 had a related patch set uploaded (by Bvibber; author: Bvibber):

[integration/config@master] Drop MobileFrontend from ReaderExperiments deps

https://gerrit.wikimedia.org/r/1240360

bvibber mentioned this in T417818: (CI) MobileFrontend test failures when used as a dep for ReaderExperiments.Wed, Feb 18, 7:35 PM

Change #1240360 merged by jenkins-bot:

[integration/config@master] Drop MobileFrontend from ReaderExperiments deps

https://gerrit.wikimedia.org/r/1240360

Change #1239411 merged by jenkins-bot:

[mediawiki/extensions/ReaderExperiments@master] Override ParserMigration state for Minerva TOC treatment and control

https://gerrit.wikimedia.org/r/1239411

Maintenance_bot removed a project: Patch-For-Review.Wed, Feb 18, 11:30 PM

ReleaseTaggerBot edited projects, added MW-1.46-notes (1.46.0-wmf.17; 2026-02-24); removed MW-1.46-notes (1.46.0-wmf.16; 2026-02-17).Thu, Feb 19, 12:00 AM

matthiasmullie moved this task from Code Review to QA on the Reader Growth Team (Sprint 3 (Feb 17 - Mar 2) Q3 25/26) board.Thu, Feb 19, 9:53 AM

Confirming that this works as expected on production now.

egardner closed this task as Resolved.Fri, Feb 20, 12:23 AM

	F71661840: Screenshot 2026-02-03 at 1.49.35 PM.png
	Feb 3 2026, 8:59 PM

	F71661838: Screenshot 2026-02-03 at 1.49.17 PM.png
	Feb 3 2026, 8:59 PM

Lead paragraph hoisting and lazy images are disabled for users in the experiment on legacy parserClosed, ResolvedPublic2 Estimated Story PointsBUG REPORTActions