Added Parsoid tag since https://en.wikipedia.org/api/rest_v1/page/html/Category:Engineering_disciplines doesn't show the categories either.

@bearND see T151223: Category listing pages are not populated properly in Parsoid

@GWicke Who is responsible for including the missing info? Is this something that MCS should provide?
What's the API call for getting the subcategories for [[Category:Engineering_disciplines]]?

For now, while the issue of how to transclude incidental info like this for Parsoid-generated pages is under discussion, any reason not to expand the app-side workaround from T135242 to load all non-mainspace pages from MW API? That would at least give users the content they expect in the meantime.

@Mholloway +1 from me

Hmm, unfortunately it seems that mobileview doesn't give us the category listings, either.

mobileview responses:

Lead: https://en.wikipedia.org/w/api.php?action=mobileview&format=json&formatversion=2&prop=text%7Csections%7Clanguagecount%7Cthumb%7Cimage%7Cid%7Cnamespace%7Crevision%7Cdescription%7Clastmodified%7Cnormalizedtitle%7Cdisplaytitle%7Cprotection%7Ceditable&onlyrequestedsections=1&sections=0&sectionprop=toclevel%7Cline%7Canchor&noheadings=true&page=Category:Engineering_disciplines&thumbsize=640

Remaining: https://en.wikipedia.org/w/api.php?action=mobileview&format=json&prop=text%7Csections&onlyrequestedsections=1&sections=1-&sectionprop=toclevel%7Cline%7Canchor&noheadings=true&page=Category:Engineering_disciplines

MCS doesn't yet fully support File and Category pages. This is something we're going to work on as part of the Page Content Service project. In the meantime I would recommend the Android app to use action=mobileview for Category pages as well.

This isn't an MCS-specific bug. It happens when loading from mobileview as well.

@Jdlrobson I'm not familiar enough with the MediaWiki internals to fully interpret what's really going on here but it looks like File pages are special-cased in ApiMobileView so that the full page content is rendered. Does something similar have to happen for Category pages and other non-mainspace pages?

(These also aren't fully rendered in MCS but that's a separate issue.)

Subcategories require JS to work. When you click on them you'll see API requests such as:

https://en.wikipedia.org/w/api.php?action=categorytree&format=json&category=Aerospace_engineering&options=%7B%22mode%22%3A0%2C%22hideprefix%22%3A20%2C%22showcount%22%3Atrue%2C%22namespaces%22%3Afalse%7D&uselang=en&formatversion=2

Removed Parsoid tags -we are correctly not shipping the HTML for those. It's perfectly fine to show just the top level categories without JS, they degrade nicely. For instance
https://en.wikipedia.org/wiki/Category:Engineering_disciplines links to the sub category https://en.wikipedia.org/wiki/Category:Aerospace_engineering however when clicked JS renders the list in place.

What's the actual problem statement here? It sounds like a case of a user expects them to show inline but actually they have to click a link to see them.

@Jdlrobson I'm not quite sure I'm following. What link are you referring to? As shown in the screenshot above, the page claims to be a list of subcategories but (in our client, when retrieved from either API) actually lists nothing. The only link I see is the external link to Commons.

Edit: I see that the categories are available there; still, it strikes me as a valid bug that they're not shown inline in the app.

Oh I have no idea how the content service behaves for categories, but it doesn't surprise me this is not working. Categories are generated from metadata and not stored in the same way as other pages - if you want to render these I recommend you add special handling for "Category:" namespace using the mediawiki apis.

It might make sense to make this a separate service in MCS. I created a service for https://trending.wmflabs.org/wiki/Special:Categories and it's subpage, however given usage is low you might be fine just using mediawiki apis. You are not blocked on services or infrastructure for this in my opinion.

Ii would suggest you use the MediaWiki API - https://en.m.wikipedia.org/wiki/Special:ApiSandbox to render categories. A read through https://github.com/jdlrobson/weekipedia/blob/master/libs/server/endpoints/categories.js will give you an idea of which apis to use. The default endpoint for page views should strictly not surface these. It will bloat the payload considerably.

Categories are now shown natively in the Android app (T73966), so untagging Android.

Adding Content-Transform-Team as Product-Infrastructure-Team-Backlog-Deprecated has been deprecated for a while, and as open valid tasks shall not be ignored and forgotten only because WMF internally reorgs without much change management in place (cf T328586).

In T153980#7872511, @Dbrant wrote:

Categories are now shown natively in the Android app (T73966), so untagging Android.

So sounds like this can be resolved?