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.

@Kipod, I'm afraid a comment on a task that got resolved 8 years ago will not have much of an effect. Nobody is actively working on this any more. CatWatch got archived. Quite a few of the subscribers here are not even in the same organisation any more.

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

two issues here.

1. the task itself is a mistake (i.e, "design bug):, IMO. when someone adds a hidden category to their watchlist, they expect to see changes (addition and removal of pages from the cat) regardless of the "show hidden cat" preference: the preference should control whether or not the hidden categories appear in the "categories" box at the bottom of the page, but not block watch items. if dev insists on the current (wrong) behavior, at the very least, these should happen:
   1. i when a user adds a hidden category to their watchlist, they should be notified that they will not see changes in the category without turning on the non-default "see hidden cat" preferences.
   2. similarly, when a user turns on the "show changes in cat" filter, they should be notified that they should be warned that they will no see changes to hidden cat.
   3. when a category is made "hidden", users who are watching it unexpectedly stop getting notified when pages are added and removed
2. the implementation itself is buggy: the bug here is that changes to hidden cats are not shown but they are counted. in the "error sate" watching a hidden cat while the preference to see hidden cats is off), if my watchlist is configured to show 100 changes, and someone does something radical which fills or empties such a category with more than 100 pages), the watchlist will show empty, i.e., the hidden cat changes push other changes out of the watchlist (i.e., they are counted), but are not shown. this behavior contradicts the normal/expected filter behavior: changes filtered out of the watchlist are not counted, and the list shows the maximum number of changes which do pass the filter.

Added in b7221eaa0893e9e1ed31355e3ea98773fbc628ba as part of T127944

We could use S to denote such edits.

Please discuss the code on Gerrit. What needs to be tested is already there

@Ammarpad are your suggestions still actual?

One of the screenshots in the bug report has been deleted from Commons and the other doesn't explain anything to me. @Antanana Do you remember what this issue was about? And does it still affect you?

Could probably do this by extending the Javascript that adds the +x to quick add/remove pages from the changes view of watchlist.

No, it's not the same problem

Yes, it is precisely the same problem. But since you don't seem interested in moving forward on the point, I'll just move on.

@Izno No, it's not the same problem or the same code issue. It's not about the last unseen diff here like you just described. It's about a convenient button in the Watchlist to see a diff of all unseen changes. I don't know how people can use the Watchlist without it, I think the Watchlist is broken. Those two issues are not about the two same thing. If there is something that they have in common create a new subtask or create a new issue where these two are subtasks but the issues themselves are very different.

The descriptions clearly discuss the same problem: I would like to know what the diff is since the last time I viewed a page. Whether that ends up being fixed with method A or B is not the point of a task, which should discuss the problem first/foremost and not necessarily specific solutions to the problem. That's why they should be merged even if I agreed that the two descriptions differ sufficiently not to be the same description of functionality intended. Which I don't. The distinction you see in your reading is not one I see in mine.

@Izno please don't close so quickly or more thoroughly compare: these are not the same – the other issue is about seeing all unseen diffs in one view like with the Watchlist showing edits directly in the Watchlist via the "Expand watchlist to show all changes, not just the most recent" option. In contrast, this issue here is about the normal Watchlist that shows one item per page but has an extra button to directly see a diff of all unseen changes for that page (one item in the Watchlist per watched article, not dozens of items per watched page),

@Ifeatu_Nnaobi_WMDE , @WMDECyn , this looks like one for your team. happy to chat about the details if needed