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

[David Kastrup <dak@gnu.org>]

Werner LEMBERG <wl@gnu.org> writes:

> [This is about behaviour of current bzr Emacs in really large
> .mew-summary files.]
>
>>> >> With bidi-dislay-reordering set to `nil', holding the the up or
>>> >> down cursor keys pressed causes a smooth scrolling, either
>>> >> forwards or backwards.  Setting it to `t', there is no smooth
>>> >> scrolling any more.  Instead, the cursor jumps, with large
>>> >> delays, to its final position.
>>> > 
>>> > If you set bidi-paragraph-direction to 'left-to-right in that
>>> > buffer, does the problem go away?
>>> 
>>> Yes.
>> 
>> Thanks.  So this is the solution for this buffer (and for any other
>> buffer whose paragraphs should always have the left-to-right base
>> direction, i.e. their lines should always start at the left margin
>> of the window).
>> 
>> As mew is not part of Emacs (I think), what would be the best way of
>> achieving this effect?  I'd prefer not to ask users to do that
>> manually, if that can be avoided.  Does this buffer have a fixed
>> name we can rely upon?
>
> Yes, the name is always `.mew-summary'.  I've CCed the mew mailing
> list so that the author of mew gets informed.  It should be
> straightforward to add a proper call to `bidi-paragraph-direction'
> within the next mew release.

I think that is a bottomless pit.  I consider it nonsensical to treat a
block of aligned lines as one large paragraph where one would need to
find out the paragraph direction of the whole.  It might be nice to have
the same default paragraph direction in the whole "paragraph" for
alignment purposes, but it is nonsensical to wait for the paragraph to
complete.

With a bit of luck, the alignment uses display properties (because
otherwise proportional fonts are raining on the parade), and in the
presence of a :space display property or what our aligning properties
were called, I think that the paragraph direction should be determined
at the end of the first line, rather than the whole "paragraph".

It might also be reasonable to set the paragraph boundary detection
regexps to nil in buffers where no paragraphs will ever be found, and
let bidi revert to a (locale-dependent?) default direction for such
buffers.

-- 
David Kastrup