@AlexeiKopylov: The priority of this task was increased. Do you plan to work on this task? If yes, please also claim the task by setting yourself as assignee. Thank you for your help!

This should not be difficult to implement. I'd be happy to review any patches.

Note that you need to patch the code in two places, for the JavaScript version of the button (code in mediawiki.special.watchlist.js which calls API action=setnotificationtimestamp – the API seems to already have an option to provide a timestamp) and for the non-JS one (code in SpecialWatchlist.php which calls User::clearAllNotifications() – that doesn't take a timestamp parameter, it'd have to be added).

By the way, if the user chose to hide some edits from the watchlist (for example by choosing a namespace) than "Mark all page as visited" should mark only those pages. (Now it seems to mark all pages).

In T98941#3334301, @AlexeiKopylov wrote:

By the way, if the user chose to hide some edits from the watchlist (for example by choosing a namespace) than "Mark all page as visited" should mark only those pages. (Now it seems to mark all pages).

That is a separate issue; please open a separate task for it.

Change 358178 had a related patch set uploaded (by Huji; owner: Huji):
[mediawiki/core@master] Watchlist "Mark all visited" synchronization

https://gerrit.wikimedia.org/r/358178

Change 358178 abandoned by Huji:
Watchlist "Mark all visited" synchronization

Reason:
Too sophisticated, not worth doing

https://gerrit.wikimedia.org/r/358178

Can this be a good task for Google-Code-in-2017?

I came here to request this and am amazed that it's been open for seven years. It's not sophisticated; it's debatably a bug fix.

In T98941#7981094, @Certes wrote:

[...] It's not sophisticated; it's debatably a bug fix.

Want to give resolving it a go? :-)

@Certes: Age is irrelevant, code contributions can help though. See https://www.mediawiki.org/wiki/Bug_management/Development_prioritization - thanks.

This task seems straightforward, but it involves both frontend and backend and also both modern and legacy interfaces. Also, things have changed a lot since T98941#3333468.

Backend work (legacy interface):

• Add optional $timestamp parameter to WatchlistManager::clearAllUserNotifications.
• Update WatchedItemStoreInterface::resetAllNotificationTimestampsForUser call to use the $timestamp parameter.
  • Question: What should be done with the TalkPageNotificationManager::removeUserHasNewMessages call? (It does not support timestamp.)
• Add a hidden timestamp field to the reset watchlist button in the legacy interface (SpecialWatchlist::setTopText).
• Read the timestamp from the request when calling WatchlistManager::clearAllUserNotifications in SpecialWatchlist.

Frontend work (legacy interface):

• Read the timestamp field in watchlist.js and include it in the action=setnotificationtimestamp API call.

Frontend work (modern interface):

• In Controller.js (markAllChangesAsSeen), include the timestamp in the action=setnotificationtimestamp API call. This interface can update without reloading the page, but the updated timestamp should be available using this.changesListModel.getNextFrom().

I think all the above should do it.