bug#43568: Horizontal mouse wheel scrolling

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Tue, 22 Sep 2020 21:40:49 +0300
> 
> As promised in 
> https://lists.gnu.org/archive/html/emacs-devel/2020-09/msg01598.html
> and mentioned in https://debbugs.gnu.org/28182#58
> this patch finally provides horizontal scrolling by shifted mouse wheel:

This is a strange use of Shift.  Is this something that other
applications or platforms do?

>  (defcustom mouse-wheel-scroll-amount
> -  '(5 ((shift) . 1) ((meta) . nil) ((control) . text-scale))
> +  '(1 ((shift) . hscroll) ((meta) . nil) ((control) . text-scale))
>    "Amount to scroll windows by when spinning the mouse wheel.
>  This is an alist mapping the modifier key to the amount to scroll when
>  the wheel is moved with the modifier key depressed.
> @@ -97,6 +97,9 @@ mouse-wheel-scroll-amount
>  a full screen to scroll.  A near full screen is `next-screen-context-lines'
>  less than a full screen.
>  
> +If AMOUNT is the symbol 'hscroll', this means that with MODIFIER,
> +the mouse wheel will scroll horizontally instead of vertically.
> +
>  If AMOUNT is the symbol text-scale, this means that with
>  MODIFIER, the mouse wheel will change the face height instead of
>  scrolling."
> @@ -123,6 +126,7 @@ mouse-wheel-scroll-amount
>                      (const :tag "Scroll full screen" :value nil)
>                      (integer :tag "Scroll specific # of lines")
>                      (float :tag "Scroll fraction of window")
> +                    (const :tag "Scroll horizontally" :value hscroll)
>                      (const :tag "Change face size" :value text-scale)))))
>    :set 'mouse-wheel-change-button
>    :version "27.1")

The :version tag should be updated.

This change, if we install it, must be documented in NEWS and in the
manual.

Thanks.