User:F-402/catwatch.js

/* Category watch (User:ais523/catwatch.js); adds entries on the watchlist if a page is  added to a category (and is still in the category when the watchlist is checked). The entry will only appear if there is another page on the watchlist edited that day (if the addition  was the only change on that day, it's neglected). The recategorisation of a page in a watched category will also trigger the watching, even if it's a different category that's added or  removed (unless the watched category is removed); removals from the watched category can't   be detected.

A previous version of this script had a limit on the maximum size of a category that could be  watched; this limit no longer exists. Thanks to User:TheDJ for helping me make this compatible with more browsers and preference settings, and to Roan Kattouw for fixing 11275 and making this cheap on the servers.

// //