bug#65070: 29.1; (Only on Windows) Both touchpad scroll gestures scroll the screen up and down.

[Eli Zaretskii]

> Cc: hiroya.ebine1@gmail.com, max@mal-richtig.de, 65070@debbugs.gnu.org
> Date: Sat, 19 Aug 2023 10:54:34 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> Po Lu, I'd appreciate if you'd take me through the code that produces
> and uses these wheel events, documented in the ELisp reference manual
> as (wheel-up/down POSITION CLICKS LINES PIXEL-DELTA).  Specifically:
> 
>   . what is the conventions for signs in the data of the PIXEL-DELTA
>     cons cell?
>   . where and how are the PIXEL-DELTA data used for scrolling?
> 
> The corresponding X code is incomprehensible for me, as it deals with
> concepts and values I cannot relate to the corresponding Windows
> functionalities.  (I think the xterm.c code which deals with
> pixel-wise scrolling could use some detailed commentary, regardless.)
> And the ELisp manual doesn't divulge enough detail, either.  So your
> help will be appreciated.

Now with Po Lu added.