bug#26599: patch for mwheel.el

[Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>]

>> >> Let’s consider a buffer consists of very long lines and an empty line.
>> >> Point starts from A with auto-hscroll-mode t.
>> >> 
>> >> 0000..0000 A
>> >> 0000..0000 I
>> >> C          B
>> >> D 00..0000 Z
>> 
>> Point reaches to B by scrolling up and as soon as auto-hscroll-mode is
>> set to t, scope will be shifted to point C.
>> 
>> I want to maintain scope with B instead of having scope with C before
>> another scrolling.
> 
> If that's what you want, why do you need to turn auto-hscroll-mode
> back on?  Just leave it off.

Yes.  However, after wheel cruise with auto-hscroll-mode nil, I want
to start editing.

Sooner or later I want to use C-a to go outside of current scope.
This is the timing when I want to set auto-hscroll-mode nil.  An event
from me should do it.  The event must be some events besides
<wheel-up>, <wheel-down>, <wheel-right>, or <wheel-left>, and can be
C-a or <down-mouse-1>.

My picture is having a minor mode with keymap <down-mouse-1>
configured to set auto-hscroll-mode t.