UAX #9: applicability of higher-level protocols to bidi plaintext

[Richard Wordingham via Unicode]

On Tue, 10 Jul 2018 19:40:59 +0300
Shai Berger via Unicode <unicode at unicode.org> wrote:

An agreement can take the form of a hidden condition that if one uses
someone else's software, one accepts what that software chooses to do.
(Most notoriously, this applies to the PUA.)  There seem to be no
applicable rules against 'unfair terms'.

> On Tue, 10 Jul 2018 13:37:56 +0200
> Philippe Verdy via Unicode <unicode at unicode.org> wrote:

> > A plain text editor should not have a default strong LTR default, it
> > should have a weak undetermined direction,  

> I agree -- but the UTC does not, according to the last entry in
> http://www.unicode.org/faq/bidi.html. I would like to convince them
> otherwise, or to be shown why my position is wrong.

Even just for horizontal text, one problem is the shape of the canvas.
If it has a left and a right-hand margin, than having an undetermined
direction by default can work, given enough memory.  The rendering
system then has to have enough memory to store the entire paragraph -
the strongly directional character may be the last one in the
paragraph.  I'm not sure that a protocol is allowed to be based on
analysing the first 100 characters of a paragraph.

However, it is common for displays to provide a window into a canvas
that is unbounded both downwards and either rightwards or leftwards.
If it is unbounded rightwards, one needs an LTR paragraph direction: if
it is unbounded leftwards, one needs an RTL paragraph direction.  I
believe that having a mix of paragraphs unbounded on the left and
paragraphs unbounded on the right would feel distinctly odd; it could
also be a challenge to manage panning the window.  It also raises the
question of where the LTR and RTL paragraphs would overlap.

Richard.