bug#43568: Horizontal mouse wheel scrolling

[Juri Linkov]

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:

diff --git a/lisp/mwheel.el b/lisp/mwheel.el
index 32fde0dd05..421bebc5f5 100644
--- a/lisp/mwheel.el
+++ b/lisp/mwheel.el
@@ -85,7 +85,7 @@ mouse-wheel-inhibit-click-time
   :type 'number)
 
 (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")
@@ -270,7 +274,11 @@ mwheel-scroll
     (condition-case nil
         (unwind-protect
            (let ((button (mwheel-event-button event)))
-             (cond ((eq button mouse-wheel-down-event)
+              (cond ((and (eq amt 'hscroll) (eq button mouse-wheel-down-event))
+                     (funcall (if mouse-wheel-flip-direction
+                                  mwheel-scroll-left-function
+                                mwheel-scroll-right-function) 1))
+                    ((eq button mouse-wheel-down-event)
                      (condition-case nil (funcall mwheel-scroll-down-function 
amt)
                        ;; Make sure we do indeed scroll to the beginning of
                        ;; the buffer.
@@ -285,7 +293,11 @@ mwheel-scroll
                           ;; for a reason that escapes me.  This problem seems
                           ;; to only affect scroll-down.  --Stef
                           (set-window-start (selected-window) (point-min))))))
-                   ((eq button mouse-wheel-up-event)
+                    ((and (eq amt 'hscroll) (eq button mouse-wheel-up-event))
+                     (funcall (if mouse-wheel-flip-direction
+                                  mwheel-scroll-right-function
+                                mwheel-scroll-left-function) 1))
+                    ((eq button mouse-wheel-up-event)
                      (condition-case nil (funcall mwheel-scroll-up-function 
amt)
                        ;; Make sure we do indeed scroll to the end of the 
buffer.
                        (end-of-buffer (while t (funcall 
mwheel-scroll-up-function)))))