Re: bidi-display-reordering is now non-nil by default

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: emacs-devel@gnu.org
> Date: Tue, 02 Aug 2011 11:34:21 -0400
> 
> I think it makes sense to change the default value of
> bidi-paragraph-direction to left-to-right.
> I expect there are fewer modes where bidi-paragraph-direction needs to
> be set to nil than modes where it needs to be set to left-to-right.
> Furthermore a left-to-right default value is safer in the sense of
> having better backward compatibility and avoiding pathological
> performance problems.

I don't necessarily disagree, and doing that is a one-line change.
But let's not do it just yet, for two reasons.

First, we only discovered 2 kinds of affected modes for now: Org mode
and the mail/news summary modes.  So our expectations of the number of
affected modes do not have enough data points to back them up.  I
think we should gather more, to be able to weigh the merits and
demerits of this.  Eventually, some modes will need to use non-default
values of this variable, so it really is an issue with how many modes
will have to do this or that change.

Second, concealing the potential performance problems at this early
stage would make the testing of the bidi code much less effective,
because evidently it was not used enough in the 1.5 years it was
available on the trunk, if such problems are only now get reported.
The problem with Org mode, for example, made it obvious that one of
the optimizations I introduced to prevent slowdowns is failing in
several important use cases, and needs to be redesigned.  If the
paragraph direction in Org buffers was forced to left-to-right, who
knows when this grave problem would be discovered and reported.

The "backward compatibility problems" part, I don't understand.  Can
you explain what you are alluding to?