Category-triggered gadgets not loaded in live preview
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	Nardog
	Oct 10 2024, 8:22 AM

Description

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

Make sure the uselivepreview preference is enabled.

Enable a gadget such as Calculator, which is defined to only load on particular category's pages:

Calculator [ResourceLoader | default | categories = Pages using gadget Calculator ] | calculator.js |calculator.css

Open a page in that category for source editing, e.g. https://en.wikipedia.org/w/index.php?title=Template:Body_roundness_index&action=edit

Click "Preview" (next to the Publish button) to load a live preview of the page.

What happens?:
The calculator does not appear.

What should have happened instead?:
The calculator appears, just as in non-live preview.

Other information (browser name/version, screenshots, etc.):
The parse API should return ext.gadget.Calculator as one of the modules.

Details

Related Changes in Gerrit:

	Subject	Repo	Branch	Lines +/-
	Add gadgets using both OutputPageBeforeHTML and BeforePageDisplay hooks	mediawiki/extensions/Gadgets	master	+66 -39
	ApiParse: initialize OutputPage with categories	mediawiki/core	master	+45 -4

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Related Objects
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Status	Subtype	Assigned	Task
Open	BUG REPORT	None	T340705 [performance budgeting] Improve JS payload for projects with gadgets that lead to a 30%+ increase after gzip
Resolved		Krinkle	T63007 Allow specifying when a gadget should load (action, namespace, content model)
Open	BUG REPORT	None	T376875 Category-triggered gadgets not loaded in live preview

Event Timeline

Nardog created this task.Oct 10 2024, 8:22 AM

Restricted Application added a subscriber: Aklapper. · View Herald TranscriptOct 10 2024, 8:22 AM

Nardog added a parent task: T63007: Allow specifying when a gadget should load (action, namespace, content model).Oct 10 2024, 8:23 AM

Ammarpad subscribed.Oct 10 2024, 1:16 PM

Xaosflux subscribed.Oct 10 2024, 7:45 PM

Bawolff subscribed.Oct 10 2024, 7:59 PM

The parse API should return ext.gadget.Calculator as one of the modules.

I believe the problem is the module presence (to be eligible for return) is dependent on category existence, but the category existence check has already happened on the first server loading of the edit page which is the only load for live preview. You can request the parsing API to return the categories of the page blindly, but then you really have to parse that info (maybe in MediaWiki-extensions-Gadgets) and dynamically load the module if it qualifies.

the category existence check has already happened on the first server loading of the edit page which is the only load for live preview

That would not be a problem. Live preview already asks for the categories as well as modules, which means the parse API is already capable of dynamically generating categories from a source. So it should just return the gadget as one of the modules if the triggering category is present in the parsed result.

The gadget in the example uses the wikipage.content hook so it loading only once is not a problem.

@SD0001 Can you look into this? The inability to live-preview template gadgets is frustrating.

The Parse API returns gadgets that have been added to the ParserOutput, not the OutputPage. The difference is that ParserOutput is agnostic of user preferences, skin, even the namespace and content model.

We could switch from BeforePageDisplay to ContentAlterParserOutput hook for category-triggered gadgets, but that comes at the cost of not being able to honour user opt-in/opt-out and the other conditional loading options.

In T376875#10470468, @SD0001 wrote:

but that comes at the cost of not being able to honour user opt-in/opt-out and the other conditional loading options.

A hack to do this would be to use RL\ClientHtml::getModules(), followed by removing the required gadgets, followed by RL\ClientHtml::setModules(). Neither OutputPage nor RL\ClientHtml expose a straightforward interface for removing modules.

This doesn't seem related to Realtime Preview as that just uses the same core feature that live preview does. I've updated the description.

SD0001 renamed this task from Parse API does not return category-triggered gadgets to Category-triggered gadgets not loaded in live preview.Aug 29 2025, 3:18 AM

Change #1195386 had a related patch set uploaded (by SD0001; author: SD0001):

[mediawiki/core@master] ApiParse: initialize OutputPage with categories

https://gerrit.wikimedia.org/r/1195386

gerritbot added a project: Patch-For-Review.Oct 11 2025, 6:08 PM

Change #1195386 merged by jenkins-bot:

[mediawiki/core@master] ApiParse: initialize OutputPage with categories

https://gerrit.wikimedia.org/r/1195386

Maintenance_bot removed a project: Patch-For-Review.Nov 3 2025, 11:30 PM

ReleaseTaggerBot added a project: MW-1.46-notes (1.46.0-wmf.1; 2025-11-05).Nov 4 2025, 12:00 AM