Re: Mixed L2R and R2L paragraphs and horizontal scroll

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 31 Jan 2010 10:36:36 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: emacs-bidi@gnu.org, emacs-devel@gnu.org
> 
>  > Yes, and they were also flushed to the right.  This is what the
>  > bidirectional display does.
> 
> With bidirectional display going to the end of the first line in the
> first window you earlier proposed to show
> 
>        +---------+---------+-------------------+
>        |$ijk     |abcdefgh$|abcdefghijk        |
>        |$HGFEDCBA|$HGFEDCBA|        KJIHGFEDCBA|
>        |         |         |                   |
>        +---------+---------+-------------------+

Yes, I did.  But now I changed my mind, see below.

> Now moving in the first window to the end of the second line do you want
> to display
> 
>        +---------+---------+-------------------+
>        |$ijk     |abcdefgh$|abcdefghijk        |
>        |     KJI$|$HGFEDCBA|        KJIHGFEDCBA|
>        |         |         |                   |
>        +---------+---------+-------------------+
> 
> or
> 
>        +---------+---------+-------------------+
>        |abcdefgh$|abcdefgh$|abcdefghijk        |
>        |     KJI$|$HGFEDCBA|        KJIHGFEDCBA|
>        |         |         |                   |
>        +---------+---------+-------------------+
> 
> or something else?

I think the former of these two is TRT.

The reason is simple, although it took me some time to arrive at it:
column numbering.  In a R2L line, columns should be numbered from the
right margin of the window.  Given the fact that a window's dimensions
can be changed at will, and thus the horizontal position of characters
in R2L lines relative to L2R lines can change, there simply isn't any
other reasonable way of numbering columns of a R2L line.  We will need
that anyway for supporting features like indentation and tab stops in
R2L paragraphs.

And since the columns are numbered from the right margin, hscroll,
too, should scroll R2L lines to the right when L2R lines are hscrolled
to the left.  That's the only reasonable way of generalizing the
current unidirectional hscroll, IMO.  It also preserves the semantics
of window-hscroll (modulo the fact that R2L lines are scrolled from
the right margin, not the left one).

Makes sense?