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

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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?

The pertinent delta is the one for the axis whose value is larger.  If
more scrolling has taken place on the X axis, then the relevant delta is
that for the Y axis.  Otherwise, it is the delta for the Y axis.

If that delta is less than 0, a wheel-down event is produced.
Otherwise, a wheel-up event is produced instead.

>>   . where and how are the PIXEL-DELTA data used for scrolling?

Only in pixel-scroll-precision-mode; the traditional mouse wheel
scrolling code derives the direction in which to scroll from the type of
the wheel event.