bug#33749: 26.1; input-decode-map to empty vector should preserve echo area

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: yurivkhan@gmail.com, 33749@debbugs.gnu.org
> Date: Tue, 25 Dec 2018 15:07:50 -0500
> 
> >> I think The Right Thing to do is likely to move this code to
> >> read_key_sequence, more specifically, move it to the point where we
> >> *know* we really do have an event.
> >> IOW, I think the patch below might be a better option (where we test
> >> `indec.start > 0` to make sure some *decoded* event was read).
> > Where in that patch is the variable that I asked for, which would
> > trigger this special behavior?
> 
> Nowhere ;-)
> 
> Mostly because I don't know where we could set this variable (other
> than globally).
> 
> If you want me to add a global var to control whether we use the old or
> the new behavior, I'm fine with that.

Yes, please.

> Note also that the current patch is likely not quite right yet anyway:
> I mostly posted it for discussion.  E.g. I think where I put it
> currently, it fails to be run for single-key commands.

Yes, I understand.