bug#29279: Sharing the margins

[Dmitry Gutov <dgutov@yandex.ru>]

On 11/13/17 5:46 PM, Eli Zaretskii wrote:
> I tend to like better the proposal originally made by Joost Kremers in
> http://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01540.html.
> Here it is with slight variations of my own:

Overall, it seems like a friendlier API over fairly similar structures 
as what I suggested (just replace left-margin-columns-alist with 
left-margin-params). Except SYMBOL is not given a meaningful name (they 
are "columns" in my description).

> We add 3 new functions:
> 
>   (defun window-margin-add (window side symbol cols &optional ordinal)
>     "Request allocation of COLS columns of WINDOW's marginal area.
>   WINDOW must be a live window and defaults to the selected one.
>   SIDE is the window side, either `left' or `right', in whose margin
>     the allocation is requested.
>   SYMBOL identifies the request, in case the allocation needs to be
>     changed or deleted.
>   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"? And the lower the value 
is, the more "left" the column should be positioned? Sounds OK to me.

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

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?

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?

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

Return value?

> 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? :)

> This keeps the display engine design intact, and I think will allow
> relatively straightforward implementation (although some non-trivial
> changes in management of the glyph matrices will be necessary).  It
> can also be implemented in a way that keeps backward compatibility
> with the current APIs.
> 
> WDYT?

Aside from the ORDINAL edge case, it sounds good to me.

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?

And if not, how can we make this API better that you could?