bug#29279: Sharing the margins

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 29279@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 13 Nov 2017 19:54:45 +0200
> 
> >   ORDINAL is the optional ordinal number of the requested area, counted
> >     from left to right.  Negative ordinal numbers count from right to
> >     left.  Zero means the value of COLS is the maximum width of the
> >     marginal area, and no separate allocation is requested.
> 
> ORDINAL meaning the same as "order priority"?

Yes.

> And the lower the value is, the more "left" the column should be
> positioned? Sounds OK to me.

Yes.

> I'm not sure I understand the "Zero means ..." passage, though.

That's your "total width" thing, for margin users that just want to
set the overall width of the margins without displaying anything
there.  Like Joost Kramer's visual-fill-column and similar packages.

> In addition to signifying a neutral position, does it supposed to switch 
> the meaning of this function into something that 
> set-right-margin/set-left-margin can call, for backward compatibility?

Yes, set-window-margins will most probably be reimplemented by calling
the above.

> Seems like a wart, using ORDINAL this way. And what's going to happen 
> when somebody else calls window-margin-add with non-zero ORDINAL? Will 
> the end result depend on which call happens first?

Yes.  And the result is returned, so the caller knows that.  If you
have better ideas for requesting a particular position in the margin,
let's hear them.

> >   Value is the actual positive ordinal number of the allocated area."
> 
> Return value?

Yes.

> > If the stuff (text, image, etc.) to be displayed in a specified
> > portion of the marginal area is too wide and doesn't fit the width of
> > the portion, it will be truncated; if it is narrower, it will be
> > padded.
> 
> Padded from the left or from the right? :)

On the right.

> Here's an interesting question: after such an API is added, will it be 
> feasible to re-implement display-line-numbers-mode using a margin 
> column, instead of the special separate area?

Yes.  But using margins from Emacs internals means that the
window-parameters which hold the column specs will change behind the
back of the Lisp applications, which I'm not sure is a Good Thing.  It
will also be somewhat slower.