bug#29279: Sharing the margins

[Eli Zaretskii]

> Date: Tue, 14 Nov 2017 10:54:36 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: 29279@debbugs.gnu.org
> 
>  >   ORDINAL is the optional ordinal number of the requested area, counted
>  >     from left to right.  Negative ordinal numbers count from right to
>  >     left.
> 
> I'd prefer ORDINAL represent an integer such that 0 means to put it in
> the middle of that margin, a negative value farther to the left and a
> positive value farther to the right with clashes decided in some
> unspecified way.

This would make it hard to request to be the leftmost or rightmost
column, I think.

> I tried a similar thing with side windows.  The idea is that a
> "don't care where to put it" application has a neutral choice
> against others which explicitly want to display their elements left-
> or rightmost.

I think this should be possible by a simple transformation of the
original argument.

> BTW is ‘window-margin-modify’ just for the sake of the line numbers
> codes or which other users would it have?

Any package whose requirement of the margin width is not constant, but
has to change due to some event.  Even visual-fill-column should need
it, I think.