I'm not sure this is the same as T8698.

Change #1107975 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] Fix paging on Special:Categories?from=… and Special:Categories/…

https://gerrit.wikimedia.org/r/1107975

Change #524652 abandoned by Bartosz Dziewoński:

[mediawiki/core@master] specials: Fix T45506 so Special:Categories understands 'offset'

Reason:

I will propose a different patch.

https://gerrit.wikimedia.org/r/524652

This doesn't seem to be the case any more, and I haven't seen it happen anywhere else. We probably won't be able to investigate what happened here 5 years ago. (I would guess that someone edited some template to add pages to that category, then reverted it.)

See T364617: Deprecate and remove alllinks API endpoint for arguments that this should not be done.

please add to the reproduction steps the following:

• add some active hidden categories to your watchlist

with all due respect, deliberate product decision does not automatically imply it's not a bug.
there are "bugs by implementation" (the more common case) and "bugs by design" (for which this is an example).

To sumarize what I wrote in T127944#10411982: This was a deliberate product decision. It's confusing to notify users about something they can't even see.

T127944 says "Categories with HIDDENCAT should not be displayed on wiki change lists unless user has the "display hidden categories" preference turned on"

Ok, see: https://www.mediawiki.org/wiki/Talk:Snippets/Hide_prefix_in_category#Hidden_category_prefix_sometimes_needs_to_be_displayed

Added in b7221eaa0893e9e1ed31355e3ea98773fbc628ba as part of T127944

Issue not tracked here. See https://www.mediawiki.org/wiki/Talk:Snippets/Hide_prefix_in_category and https://www.mediawiki.org/wiki/MediaWiki_talk:Common.js instead.

It could be easy to exclude categories with __HIDDENCAT__ from the check and track pages/files with only hiddencat as uncategorized. Not sure if the database query is to expensive, but that does not need a new magic word.

I'm moving this to watching / waiting on the Search Platform side. It seems that some work needs to happen to publish dumps before we need to be involved. Please ping us (or create a subtask) when our involvement is needed!