Added in b7221eaa0893e9e1ed31355e3ea98773fbc628ba as part of T127944

hmmmm....

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

it's not clear where did this requirement come from. i do not think it's right: the fact a user did not choose to see hidden categories in articles and other pages, does not mean they do not want to watch categories they chose to watch.
if any "special treatment" is due hidden categories in watchlist, the way to implement it is by splitting the "watch changes in categories" filter to "watch changes in visible cats/watch changes in hidden cats".
absence of changes in categories from watchlist when selecting to display changes in cats is perceived by users as a bug, and IMO it is indeed a bug (albeit "by design").

worse yet, there is an undisputable bug in the implementation of this (bad/wrong) feature:
even though the changes in hidden cats are not shown, they _are_ counted against the "show XXX last changes" selection.
IOW, if i choose to display 100 latest changes and show changes in categories, and it so happen that the 100 most recent changes in my watchlist are all additions/removals of pages from some hidden cat (something that can easily happen in real life, e.g. when using a tool like cat-a-lot, or making some change in a template which adds or removes large number of pages from some maintenance hidden category), the watchlist will show empty: those 100 "filtered out" changes push out all the other changes from the watchlist.

calling the originator of T127944, @Danny_B . also adding them to the "subscribers" list of this story. apologies if i shouldn't have done that.

peace.

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.

The counting bug mentioned later should be described in a separate ticket.

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).

will not harp indefinitely, and once you dismiss this plea i'll leave it alone, but i want to make the case at least once, trying to convince this deliberate product decision is wrong.

i will present two "ad absurdum" similar product decisions, which i hope we can all agree would be wrong, and try to make the case this one is just as wrong.
both relate to a user who did not change their preferences and hence do not see hidden categories at the bottom of pages.

1. proposed product decision: when such user opens a hidden category page, display blank category, and hide all the pages/subcats in it
2. proposed product decision: when hidden category A belongs in itself to some other, non-hidden category B (in wiki lingo: A is a sub-category of B), do not show A in the categories section of B, and instead show only non-hidden categories if there are any, or nothing if there are none.

i think we can all agree that those proposals would be wrong and harmful: for (1), when i open a category page i expect to see the pages (and sub-cats) in it, regardless of my preference choice to not see hidden categories and regardless of whether this category itself is marked as "hidden". for (2), when i open a category page, i expect to see all the categories in it, regardless of whether _they_ are hidden.

similarly, the "deliberate product decision" discussed here is wrong, for exactly the same logic: when i choose to watch some category, i expect to see changes to it in my watchlist, regardless whether it's hidden or not. if i didn't want to see these changes i would not watch this category.

i realize it's not trivial to re-review a deliberate product decision that was taken some nine years ago (possibly not everyone who participated in this decision is even around nine years later), but i think you should re-review it and revert this illogic, wrong and harmful decision.

peace.

In T382217#10410747, @Kipod wrote:

[...]

worse yet, there is an undisputable bug in the implementation of this (bad/wrong) feature:
even though the changes in hidden cats are not shown, they _are_ counted against the "show XXX last changes" selection.
IOW, if i choose to display 100 latest changes and show changes in categories, and it so happen that the 100 most recent changes in my watchlist are all additions/removals of pages from some hidden cat (something that can easily happen in real life, e.g. when using a tool like cat-a-lot, or making some change in a template which adds or removes large number of pages from some maintenance hidden category), the watchlist will show empty: those 100 "filtered out" changes push out all the other changes from the watchlist.

For the record, a task for this issue was created & is tracked as T382553: Hidden categories take up space in both the watchlist and the recent changes list when the option "preferences > appearance > show hidden categories" is turned off

Related discussion/bug-report at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Watchlist_category_changes_not_being_shown