Scroll right and left using wheel-right and wheel-left.

These changes also make use of touchpad and trackpad (Bug#26347).

* doc/emacs/frames.texi (Mouse Commands): Document horizontal
scrolling using the mouse wheel.

* lisp/mwheel.el (mwheel-scroll): Respond to wheel-right and wheel-left.
(mwheel-tilt-scroll-p, mwheel-flip-direction)
(mwheel-scroll-left-function, mwheel-scroll-right-function): New
defcustoms.
(mouse-wheel-left-event, mouse-wheel-right-event): New variables,
events that calls wheel-left/right.

* etc/NEWS: Mention horizontal scrolling using the mouse wheel.

doc/emacs/frames.texi

@@ -207,6 +207,15 @@ buffers are scrolled.  The variable
 @code{mouse-wheel-progressive-speed} determines whether the scroll
 speed is linked to how fast you move the wheel.
 
+@vindex mwheel-tilt-scroll-p
+@vindex mwheel-flip-direction
+Emacs can also support horizontal scrolling if your mouse's wheel can
+be tilted.  This feature is off by default; the variable
+@code{mwheel-tilt-scroll-p} turns it on.  If you'd like to reverse the
+direction of horizontal scrolling, customize the variable
+@code{mwheel-flip-direction} to a non-@code{nil} value.
+
+
 @node Word and Line Mouse
 @section Mouse Commands for Words and Lines
 

etc/NEWS

@@ -330,6 +330,12 @@ substituted by a home directory by writing it as "/foo:/:/~/file".
 settings of 'scroll-margin' up to half the window size, instead of
 always restricting the margin to a quarter of the window.
 
++++
+** Emacs can scroll horizontally using mouse, touchpad, and trackbar.
+You can enable this by customizing 'mwheel-tilt-scroll-p'.  If you
+want to reverse the direction of the scroll, customize
+'mwheel-flip-direction'.
+
 +++
 ** Emacsclient has a new option -u/--suppress-output.  The option
 suppresses display of return values from the server process.

lisp/mwheel.el

@@ -187,8 +187,8 @@ This can be slightly disconcerting, but some people prefer it."
 
 (defun mwheel-scroll (event)
   "Scroll up or down according to the EVENT.
-This should be bound only to mouse buttons 4 and 5 on non-Windows
-systems."
+This should be bound only to mouse buttons 4, 5, 6, and 7 on
+non-Windows systems."
   (interactive (list last-input-event))
   (let* ((selected-window (selected-window))
          (scroll-window
@@ -250,6 +250,16 @@ systems."
                  (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)))))
+                ((eq button mouse-wheel-left-event) ; for tilt scroll
+                 (when mwheel-tilt-scroll-p
+                   (funcall (if mwheel-flip-direction
+                                mwheel-scroll-right-function
+                              mwheel-scroll-left-function) amt)))
+                ((eq button mouse-wheel-right-event) ; for tilt scroll
+                 (when mwheel-tilt-scroll-p
+                   (funcall (if mwheel-flip-direction
+                                mwheel-scroll-left-function
+                              mwheel-scroll-right-function) amt)))
 		(t (error "Bad binding in mwheel-scroll"))))
       (if (eq scroll-window selected-window)
 	  ;; If there is a temporarily active region, deactivate it if
@@ -295,7 +305,7 @@ the mode if ARG is omitted or nil."
         (global-unset-key key))))
   ;; Setup bindings as needed.
   (when mouse-wheel-mode
-    (dolist (event (list mouse-wheel-down-event mouse-wheel-up-event))
+    (dolist (event (list mouse-wheel-down-event mouse-wheel-up-event mouse-wheel-right-event mouse-wheel-left-event))
       (dolist (key (mapcar (lambda (amt) `[(,@(if (consp amt) (car amt)) ,event)])
                            mouse-wheel-scroll-amount))
         (global-set-key key 'mwheel-scroll)
@@ -307,6 +317,45 @@ the mode if ARG is omitted or nil."
   "Enable mouse wheel support."
   (mouse-wheel-mode (if uninstall -1 1)))
 
+
+;;; For tilt-scroll
+;;;
+(defcustom mwheel-tilt-scroll-p nil
+  "Enable scroll using tilting mouse wheel."
+  :group 'mouse
+  :type 'boolean
+  :version "26.1")
+
+(defcustom mwheel-flip-direction nil
+  "Swap direction of 'wheel-right and 'wheel-left."
+  :group 'mouse
+  :type 'boolean
+  :version "26.1")
+
+(defcustom mwheel-scroll-left-function 'scroll-left
+  "Function that does the job of scrolling left."
+  :group 'mouse
+  :type 'function
+  :version "26.1")
+
+(defcustom mwheel-scroll-right-function 'scroll-right
+  "Function that does the job of scrolling right."
+  :group 'mouse
+  :type 'function
+  :version "26.1")
+
+(defvar mouse-wheel-left-event
+  (if (or (featurep 'w32-win) (featurep 'ns-win))
+      'wheel-left
+    (intern "mouse-6"))
+  "Event used for scrolling left.")
+
+(defvar mouse-wheel-right-event
+  (if (or (featurep 'w32-win) (featurep 'ns-win))
+      'wheel-right
+    (intern "mouse-7"))
+  "Event used for scrolling right.")
+
 (provide 'mwheel)
 
 ;;; mwheel.el ends here