I recently refactored SpecialWatchlist and SpecialRecentChanges to bring them closer to 2014, creating a base class ChangesListSpecialPage for dealing with pages like these. The refactoring is available as 27 patches on gerrit, 19 of them still waiting to be merged: https://gerrit.wikimedia.org/r/#/q/project:mediawiki/core+branch:master+topi...

This brings some consistency between the two special pages (whose implementations were diverging since 2006), including some new hooks which you're going to love. There are, as far as I am aware, no breaking changes in the internal APIs (yet; I'd like to remove some cruft, but I'm afraid of causing too many fatals ;) ) nor in the user interface.

This should ease some pain on Wikidata and Flow developers (all of whom do or want to do some ungodly things to watchlists), MobileFrontend developers (who will hopefully abandon the thought that a separate SpecialMobileWatchlist class is a good idea ;) ) and designers hoping to do some UI work on these specials (which would previously have to be implemented from scratch twice). A few other extensions use the related hooks, too: http://p.defau.lt/?VyhiMoZtYZsfrKsotU95Tg .

Below is a list of currently pending changes in stack order for your convenience. Reviews and merges (most of these are trivial) very much appreciated. (Note that issues in earlier patches are often corrected in latter ones, especially those marked with 'todo'.)

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https://gerrit.wikimedia.org/r/#q,9c98cb3,n,z Rename Watchlist request parameters for consistency with RC's ones https://gerrit.wikimedia.org/r/#q,02ba62d,n,z SpecialWatchlist: Use FormOptions for parameter handling https://gerrit.wikimedia.org/r/#q,85ff221,n,z SpecialWatchlist: Don't overwrite context now that we don't have to https://gerrit.wikimedia.org/r/#q,b7093f5,n,z SpecialWatchlist: Reorder some stuff in #execute https://gerrit.wikimedia.org/r/#q,64141c4,n,z SpecialWatchlist: Synchronise some code with newer versions from RC https://gerrit.wikimedia.org/r/#q,d52909c,n,z SpecialWatchlist: JS enhancements to namespace selector (like RC) https://gerrit.wikimedia.org/r/#q,2bf87b7,n,z SpecialWatchlist: Split #execute into subfunctions like SpecialRecentChanges https://gerrit.wikimedia.org/r/#q,e42520b,n,z Always load 'mediawiki.special.changeslist' on SpecialRecentChanges and subclasses https://gerrit.wikimedia.org/r/#q,b3ed7e9,n,z Create ChangesListSpecialPage as a base class for Watchlist and RC https://gerrit.wikimedia.org/r/#q,e8a2298,n,z Changes list legend modules cleanup https://gerrit.wikimedia.org/r/#q,2f089b8,n,z ChangesListSpecialPage and subclasses: Reorder functions https://gerrit.wikimedia.org/r/#q,8b8f35d,n,z Change behavior of Special:Watchlist when user's watchlist is empty https://gerrit.wikimedia.org/r/#q,6cd99e7,n,z No longer display the number of rows shown on Special:Watchlist https://gerrit.wikimedia.org/r/#q,48da066,n,z ChangesListSpecialPage: Implement execute() https://gerrit.wikimedia.org/r/#q,f402316,n,z ChangesListSpecialPage: Implement buildMainQueryConds() https://gerrit.wikimedia.org/r/#q,c7aac1b,n,z ChangesListSpecialPage: Implement doMainQuery() https://gerrit.wikimedia.org/r/#q,8f01c9a,n,z ChangesListSpecialPage: Implement webOutput() https://gerrit.wikimedia.org/r/#q,f366add,n,z ChangesListSpecialPage: Stop mutating $opts in buildMainQueryConds() https://gerrit.wikimedia.org/r/#q,94f2465,n,z ChangesListSpecialPage: Implement two new hooks superseding 4 old ones