bug#68213: 30.0.50; completion-preview-tests failure in --without-x build

[Drew Adams]

> It also gets rid of `mwheel-event-button` which has been obsolete for
> the last 20 years, apparently.  This is notable, because
> `mwheel-event-button` is the only part of the code that distinguishes
> between `mouse-wheel-*-event` and
> `mouse-wheel-*-alternate-event`, AFAICT.

I'm not following this thread.  But I did
happen to see that bit.  I have code that uses
`mwheel-event-button', so I'd need to know what
to replace it with.

E.g., I have this `cond' test:

((and (consp evnt)
      (member (event-basic-type (car evnt))
              `(,wheel-up   ,wheel-down
                ,wheel-left ,wheel-right)))
 (let ((button  (mwheel-event-button evnt)))
   (cond ((memq button (list wheel-down wheel-left))
          (setq new-incr  incr))
         ((memq button (list wheel-up   wheel-right))
          (setq new-incr  (if (atom incr)
                              (- incr)
                            (mapcar #'- incr))))

BTW - I don't have any Emacs 30 build, but in Emacs
29.1.2 (and earlier releases) I see nothing about
`mwheel-event-button' being "obsolete".  Why do you
say that's been the case for 20 years now?