(In reply to comment #0)
> @Matma.rex: You mean the fact that the deprecated mwCustomEditButtons global
> (unlike the mw.toolbar interface) is inspected only once by
> mediawiki.action.edit.js, namely at the domready event. And if it is
> triggered
> before the user script, adding to that array will become a no-op.

Yes, precisely.


> Though there is a lot of usage of this feature, lets not forget the context:

> * The mwCustomEditButtons interface is for the classic toolbar (aka "Monobook
> editor", though Vector users can also disable WikiEditor to get to it). This
> is
> already a large minority of users.

Yeah, my guesstimate is a few hundred power-users across all of the Wikimedia wikis. (I just searched for 'mwCustomEditButtons' in user pages on a few large projects, and tried to account for inactive ones.)


> * The mwCustomEditButtons interace has been deprecated in favour of
> mw.toolbar.addButton, which I created especially for the legacy toolbar users
> last year to address the many declined bug reports for mwCustomEditButtons
> which could not be otherwise addressed because mwCustomEditButtons is beyond
> repair, it is a flawed concept and it keeps breaking for many reasons.
> 
> I don't think anything related to mwCustomEditButtons can justify a change of
> any kind unless it comes at absolutely no overhead and downside.

I realize that this is deprecated, and agree that it's broken by design. But killing it should be preceded by some release notes and some Wikimedia announcements (or just fixing users' JavaScript for them, since there are comparatively few of them).

I have reopened the previous bug because of the combination of Liangent's comment (which I didn't verify myself) and at least two users complaining about this on #wikimedia-tech today, including one who was kind and persistent enough to help me confirm that this was what broke the functionality ([[nl:User:Romaine]]).

So I'm all for removing this legacy compatibility, but let's do it right instead of as a side-effect of an unrelated fix.