bug#29279: Sharing the margins

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 29279@debbugs.gnu.org, joostkremers@fastmail.fm
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 21 Nov 2017 00:23:26 +0200
> 
> > What is the difference between being notified when the width changed,
> > and figuring out when it was changed by comparing two numbers?
> 
> The latter approach requires more user code.

Yes, but not a lot more, and the code is fairly straightforward.

> I'd say it's a bit like the difference between having a 
> post-self-insert-hook and not having it, asking users to rely on 
> post-command-hook.

You know me well enough: if the hook you are asking for were easy to
implement safely, it would have been written long ago.

> I think we've collected the requirements by now, though. Time for an
> implementation?

Yes.