@Jdlrobson When you pinged me about this (before the task existed) I found a couple of things that might be useful. It's inconclusive though, but it might help get you closer to the culprit.

Using the same browser configuration and browsing around the wiki, I was also able to reproduce this. For me it happened at https://en.m.wikipedia.org/wiki/Cathedral_glass. And I can consistently reproduce it there now.

For convenience, I also tried reproducing it on desktop and while it doesn't reproduce in Chrome (macOS), it does reproduce on regular desktop Safari 12.0 (macOS 10.13) as long as "Developer > User Agent > Safari - iPhone" is set. (In all cases, using the m-dot domain in the url.)

Which means it probably isn't specific to how iOS or Mobile Safari behave, but rather might be specific to how our code behaves when given a User-Agent string. Maybe that'll help you find the underlying cause?

Artefacts

Below are a couple of artefacts I captured that might help with debugging. The following modules queued from the cached HTML in Varnish for the above url:

html/head/modules

RLPAGEMODULES=["mediawiki.page.startup","mediawiki.user","skins.minerva.watchstar","skins.minerva.editor","skins.minerva.options","skins.minerva.scripts.top","skins.minerva.scripts","skins.minerva.toggling","ext.gadget.switcher","ext.centralauth.centralautologin","ext.visualEditor.targetLoader","mobile.site","mobile.init","ext.relatedArticles.readMore.bootstrap","ext.eventLogging.subscriber","ext.wikimediaEvents","ext.navigationTiming","ext.centralNotice.geoIP","ext.centralNotice.startUp"];mw.loader.load(RLPAGEMODULES);});

Parser cache timestamp, and web server that originally populated it last month:

html/body/parseroutput

"cachereport":{"origin":"mw1327","timestamp":"20181014231805","ttl":1900800,"transientcontent":false}

Web server that served it in the last 24 hours and Varnish layers that cached and re-used it since:

http/Response headers

server: mw1265.eqiad.wmnet
x-cache: cp1083 hit/1, cp3040 hit/4, cp3033 miss
x-cache-status: hit-local

Duplicate module

This might be a red-herring, but I noticed part of the mediawiki.template.muhogan code might not be working as intended.

1. The QuickSurveys and RelatedArticles extensions both specify a module called mediawiki.template.muhogan in extension.json. The module registry is an array in PHP, which means only one of these will exist. I don't know which one wins in production, but they have logically identical contents so the error can come from either, and both will need to be patched in a similar way. (Note that which one wins may differ between prod, beta and local dev.)
2. The implementation from RelatedArticles (source) is actually slightly different in that it has a bit of extra logic that tries to obtain the compiler from QuickSurveys first. It falls back gracefully, but per point 1, the extra bit is redundant because the two modules are mutually exclusive.

Next steps

What I've got so far is that:

• mediawiki.template.muhogan has a declared dependency on either mustache or hogan. This is implemented server-side in ResourceLoaderMuHoganModule::getDependencies by varying the return value based whether target === 'mobile'. I've confirmed that this works and makes its way to the browser without issue, by inspecting mw.loader.moduleRegistry['mediawiki.template.muhogan'] in Safari on the problematic page and I see it does include dependencies:["mediawiki.template.hogan"].
• By the time its script executes, one or those modules will always be "ready" – given ResourceLoader dependency resolution.
• Somehow, when the user-agent is Mobile Safari, neither is present via mw.template.getCompiler despite one of two modules having reached state "ready".

Based on this I would conclude that:

1. mediawiki.template.hogan is failing to (consistently) register the hogan compiler as part of its initial script execution. Rather, it sometimes happens only later based on some external factor.
2. This external factor seems to vary its behaviour based on the user agent.

I haven't found the external factor, but I have confirmed the first conclusion. Inspecting mw.loader.moduleRegistry['mediawiki.template.muhogan'].script shows that its implementation contains the following:

MobileFrontend/mediawiki.template.hogan

this[void 0]=(window.webpackJsonp=window.webpackJsonp||[]).push([[2],{/* ....... */ 52:function(n,t,r){var e=r(53);mw.template.registerCompiler("hogan",e)},53:function(n,t,r){var e=r(14);n.exports={compile:function(n){return e.compile(n)}}}},[[52,0]]]);

Which means that implementing "mediawiki.template.hogan" basically just consists of the following two statements:

window.webpackJsonp = [];
window.webpackJsonp.push( ... );

The transported code that calls mw.template.registerCompiler() is not executed! (It is only stored as a function inside an array.)

Presumably there is another piece of code somewhere that defines the real webpackJsonp and calls the pushed functions. However, that code is not a dependency of mediawiki.template.hogan.

Presumably there is yet another piece of code that, on desktop UAs and on most pages for mobile UAs, happens to trigger this earlier.

I'll leave it to you to investigate further. Good luck :)

Mmm interesting.. I wonder if https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/MobileFrontend/+/469153/ will take care of this when it gets deployed...

In T208605#4717017, @Jdlrobson wrote:

Mmm interesting.. I wonder if https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/MobileFrontend/+/469153/ will take care of this when it gets deployed...

^^ Yeah my bet is that the webpack runtime code isn't being loaded in time (which I think that patch will address)

In which case this might get fixed in 1.33.0-wmf.2 (and we'll know Monday)

There are 2 templates:

<strong>{{question}}</strong>
<p>{{description}}</p>
<div class="survey-button-container"></div>
<div class="survey-footer">{{{footer}}}</div>

<strong>{{finalHeading}}</strong>
<div class="survey-footer">{{{footer}}}</div>

Switching these to jQuery would be pretty trivial.

Change 495903 had a related patch set uploaded (by Jdrewniak; owner: Jdrewniak):
[mediawiki/extensions/QuickSurveys@master] Removes Muhogan module & templates in favor of jQuery

https://gerrit.wikimedia.org/r/495903

Change 495903 merged by jenkins-bot:
[mediawiki/extensions/QuickSurveys@master] Removes Muhogan module & templates in favor of jQuery

https://gerrit.wikimedia.org/r/495903