bug#11680: flyspell should use mouse-3 instead of mouse-2

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Wed, 12 Aug 2020 23:10:11 -0700
> Cc: l26wang@gmail.com, 11680@debbugs.gnu.org
> 
> > I'm okay with making this conditional on some user option, but I
> > object to an unconditional change.
> 
> That is a good solution, if the default is to enable it.  That would
> allow users to easily switch back if they don't like the new behaviour.

I think it should be an opt-in behavior, conditional on a user
option.  We can later see if it is popular enough to make that the
default behavior.

Thanks.