bug#26347: patch for mwheel.el

[Eli Zaretskii <eliz@gnu.org>]

> From: Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>
> Date: Wed, 5 Apr 2017 08:18:42 +0900
> Cc: Kunihiro Tak <tkk@misasa.okayama-u.ac.jp>,
>  rudalics@gmx.at,
>  26347@debbugs.gnu.org
> 
> Now I leaned auto-hscroll-mode is irrelevant to horizontal scroll.
> However, I still insist that it should be off on horizontal and
> vertical scrolling.
> 
> (A) It should be off during vertical scroll.  Let’s consider a buffer
>    is with short and long alternative lines and when point is at the
>    end of long line, at the top of current window.  After `scroll-up
>    1', point jumps to the end of the next short line and you see scope
>    shifts suddenly leftward. This behavior is sometimes unexpected
>    one.
> 
> (B) It should be off during horizontal scroll.  During horizontal
>    scroll, you may scroll a little in vertical direction without
>    intention.  The horizontal scroll should be tolerance against such
>    perturbation.  The source of concern is same as (A).
> 
> The concern (A) is shared with vertical scroll-bar.  With this
> respect, the concern should be solved by `scroll-up' not by
> `mwheel-scroll'.
> 
> I think when point moves off the left edge of the window with cursor
> frozen at the left edge of the window, `scroll-up' should scroll up
> without horizontal shift.
> 
> Conclusion: Yes.

The considerations you present are not special to the proposed
horizontal scrolling using a mouse wheel, they are valid for any
scrolling.  IOW, this is how scrolling behaved in Emacs since about
forever.  And I think it's not a good idea to have some methods of
scrolling behave differently from others.  So I could agree to having
a user option to temporarily disable auto-hscroll-mode during
scrolling, but the default behavior should be what we have now, and
the option should affect all methods of scrolling alike.

Thanks.