A local run of HEAD (as of Jan 17, I'm at daec3e) with
"./configure --with-pgtk" has mouse-wheel-up-event set to wheel-down
because (featurep 'pgtk-win) is t, even with -nw on the command line.
Testing on Windows at Emacs 29.1 shows the same behavior.
This has been changed on `master` around Jan 19. Now `mwheel.el` uses
`wheel-up/down` unconditionally and `mouse-wheel-up/down-event` is only
used for *other* events (defaults to `mouse-4/5`).
Also, I do not think xt-mouse knows how to generate mouse-6 or mouse-7
events. I think to generate events beyond mouse-5, you need to test
against the 128 bit as well. I don't have mouse hardware to confirm
this behavior.
I didn't both to check the code to see if that can happen (especially
since there are various possible encodings coming from the terminal).
The way wrote the code, it'll do the right thing if/when the rest of
`xt-mouse` manages to generate those events, but indeed it might be the
case that currently this will never happen.