bug#69384: 30.0.50; :align-to and bidi-paragraph-direction interaction

[Stephen Berman via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Sun, 25 Feb 2024 19:35:22 +0200 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: 69384@debbugs.gnu.org
>> Date: Sun, 25 Feb 2024 18:17:35 +0100
>>
>> >> Another alternative to get this alignment is to leave
>> >> bidi-paragraph-direction at its default value of nil and use a space
>> >> display specification with a suitable value for the :align-to property,
>> >> but this requires a calculation that seems to depend at least on the
>> >> font used and the values of `window-width' and `string-width' for the
>> >> string used (or the corresponding pixel-width values); see the second
>> >> sexp below and the middle buffer in the attached screenshot.
>> >
>> > Yes, because in a RTL paragraph, columns are counted from the right
>> > edge of the window.  So this again is the expected behavior, and if
>> > you consider that unexpected, please explain why.
>>
>> This is also what I expected.  But it would be nice if there were an
>> easier way to calculate the value of :align-to (I looked in the Elisp
>> manual but didn't find anything that would give me the value 0.75 I
>> found by trial and error).
>
> I don't understand well enough what were you trying to achieve, so I
> cannot suggest how to do that.

I want to display RTL text aligned with LTR text; the buffer contains
mainly LTR text.  Currently just setting bidi-paragraph-direction to
'left-to-right is sufficient for aligning at column 0, but I may want to
align to some column in the middle of the window, which I why I tried
using :align-to and found calculating the needed value cumbersome.

>> >> It's also possible to avoid this calculation and use an :align-to value
>> >> of 0 in combination with setting bidi-paragraph-direction to
>> >> 'left-to-right;
>> >
>> > What is the purpose of using ":align-to 0"? what do you think is the
>> > semantics of that and your expectations?
>>
>> I wanted to align to the first column (0) in LTR order, for which it's
>> necessary set bidi-paragraph-direction to 'left-to-right.  The resulting
>> alignment is what I wanted and expected, but the broken shaping is not.
>
> What do you mean by "first column (0) in LTR order"?  D|o you mean the
> leftmost column in the visual order? something else?

Yes, column 0 as in a buffer containing only LTR text.

>> > Still, I don't think I understand what are you reporting as a bug
>> > here.  Please clarify.
>>
>> Sorry for not being clearer in my report: the bug is the broken shaping
>> with the combination of setting bidi-paragraph-direction and using
>> :align-to
>
> OK, I will look into that when I have time.

Thanks.

>> the first two examples were meant to show that using these
>> individually also works.  But I gather from your reply that I've
>> misunderstood what :align-to set to 0 means; can you enlighten me?
>
> It's supposed to be a no-op, of course.  Exactly like in the strict
> LTR unidirectional text.  What else can it mean?

I now realize I shouldn't have used :align-to 0 for this case, since
setting bidi-paragraph-direction to 'left-to-right already gives the
desired column-0 alignment.  It's for alignment to a columns > 0 that
:align-to in combination with bidi-paragraph-direction could be useful,
if the value of :align-to were easy to use, e.g. simply to to align the
RTL text to LTR-column 20 (instead of having to do some trial-and-error
calculation).  But the broken shaping thwarts that.

Steve Berman