bug#5557: <left-margin> <double-wheel-down> is undefined

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Tue, 1 Oct 2019 19:39:17 +0200
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 5557@debbugs.gnu.org, 
> 	Lennart Borgman <lennart.borgman@gmail.com>
> 
> FWIW, I personally think it makes sense to add this binding also to
> the fringes and the scroll bar.  If the user tries to use the mouse
> wheel there, it's it my opinion most likely just a case of missing the
> window by a couple of pixels.

Maybe.  I'd like to hear more opinions, because we didn't have that
since Emacs 21 till now.

> > I'm guessing that the OP wanted to have the display scrolled no matter
> > where on display the user turns the mouse wheel, but evidently that's
> > not how things are defined by default.  If we decide to change that,
> > it IMO makes little sense to do that only for the margins, but not for
> > the fringes or the scroll bar.
> 
> I'm not seeing this when I use the mouse wheel on the scroll bar on
> GTK 3.  Instead, the "bar" (inside the scroll bar) that indicates the
> current position strangely moves downwards slowly.  I get no error
> message, and the window doesn't scroll.

That's because in your build these gestures on the scroll bar are
processed by GTK, not by Emacs.  Try some other toolkit, or maybe
no-toolkit, and you will see what I see.

But you do see what I described on the fringes, right?