Re: Better handling of window margins

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Thu, 03 Dec 2015 15:43:34 -0500
> 
> > My suggestion wasn't supposed to do anything about post-command-hook.
> > It aimed only at allowing separate features display in the margins
> > without interfering with one another.
> 
> WRT to sizing the margin, I think if we want to "do it right" it'd make
> sense to provide an Elisp-layer that defines two functions which could
> look like:
> 
>    (window-margin-register OWNER SIZING-DESCRIPTOR)
> 
> and
> 
>    (window-margin-unregister OWNER)
> 
> so those two functions can appropriately combine the SIZING-DESCRIPTORS
> still active.

Not sure why window-margin-register and window-margin-unregister
couldn't be set-window-margins with additional arguments.

> Not sure what SIZING-DESCRIPTOR should look like (it could be
> a function which takes a size and returns a new size).

I don't think such a function can be implemented by any particular
mode, because a mode only knows well what it needs from the margins,
it has no better idea about other modes' needs than the rest of Emacs.

Given the SIZING-DESCRIPTOR I suggested, it's trivial to determine how
wide the margins should be at any given time in order to accommodate
the needs of all the modes that requested screen estate in the
margins.  So we can do that without asking modes to provide their
functions for it.  To say nothing of the fact that having more than
one function again raises a problem of in what order to apply them,
and what would be the results of applying conflicting functions in any
order.