Change 610025 had a related patch set uploaded (by Bartosz Dziewoński; owner: Esanders):
[mediawiki/extensions/VisualEditor@master] Edit API: Run OutputPageBeforeHTML hook on output

https://gerrit.wikimedia.org/r/610025

This seems a bit scary, because this hook is normally ran inside OutputPage, and OutputPage does a bajillion different things, and other extensions using this hook can depend on any of them.

But I set out to investigate and it's not as scary as it seems:

Here are all the extensions enabled on Wikimedia wikis that use this hook:

• DoubleWiki: Used on Wikisource, e.g. https://en.wikisource.org/wiki/A_Study_in_Scarlet/Part_1/Chapter_1?match=fr. It only kicks in when the 'match' parameter is used, so it shouldn't be affected.
• LiquidThreads: Still used on some wikis, e.g. https://hu.wikipedia.org/wiki/Szerkesztő:Matma_Rex/lqt. Looks like this patch would actually fix a bug where after editing a page containing a LQT board with VE/NWE, the board would not appear – except that you can't launch VE/NWE on them because LQT suppresses the normal edit links, and when launching via action=edit, VE will reload the whole page anyway. Overall it shouldn't be affected.
• MobileFrontend: Only uses the hook when mobile mode is active, and mobile mode VE reloads the whole page after saving. It shouldn't be affected until we make some changes for T219420.
• ShortUrl: Used on some non-Latin-script wikis, e.g. https://hi.wikipedia.org/wiki/सदस्य_वार्ता:Matma_Rex. The hook doesn't even do anything with the HTML, it just adds a RL module to the page, it shouldn't be affected. (As a side note, there's a different bug in its interaction with VisualEditor: after a page is edited, the link icon added by this extension next to the page title disappears.)
• Wikibase: Their code should only do stuff on entity pages, which can't be edited with VE anyway.

Overall, it seems that nothing terrible should happen…

There are ~15 other extensions not used on Wikimedia wikis that appear to use this hook, I didn't review their code. I'm hopeful that they also don't do anything ridiculous, and either won't be affected, or this will actually fix issues like in LiquidThreads.

Change 610025 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] Edit API: Run OutputPageBeforeHTML hook on output

https://gerrit.wikimedia.org/r/610025

I don't know what you would even test here, other than confirming that nothing is affected. But we should watch for any bug reports related to these extensions.

In T258980#6339131, @matmarex wrote:

I don't know what you would even test here, other than confirming that nothing is affected. But we should watch for any bug reports related to these extensions.

Per a conversation with @Esanders yesterday, it sounds like the risk of the above will go up once T252555 is merged; I've indicated as much on that ticket: T252555#6345275.