GNU bug report logs - #41626
28.0.50; activating mark breaks RTL shaping in etc/HELLO (auto-composition-mode)

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 28.0.50; activating mark breaks RTL shaping in etc/HELLO
 (auto-composition-mode)
Date: Sun, 31 May 2020 10:02:40 +0000

[Message part 1 (text/plain, inline)]

Recipe:
start emacs -Q etc/HELLO
move point to character position 358 (ARABIC LETTER SEEN)
hit C-SPC to activate the mark

Expected result:
mark is activated, no change to shaping

Actual result:
The character at position 357 is no longer composed with the one at
position 358. (screenshot with-mark.png) whereas it was composed
before (screenshot without-mark.png)

The same thing works fine in a separate buffer in an RTL paragraph.

[without-mark.png (image/png, attachment)]

[with-mark.png (image/png, attachment)]

Message #8 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: "Basil L. Contovounesios" <contovob <at> tcd.ie>
To: Pip Cet <pipcet <at> gmail.com>
Cc: 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50; activating mark breaks RTL shaping in
 etc/HELLO (auto-composition-mode)
Date: Sun, 31 May 2020 13:40:57 +0100

Pip Cet <pipcet <at> gmail.com> writes:

> Recipe:
> start emacs -Q etc/HELLO
> move point to character position 358 (ARABIC LETTER SEEN)
> hit C-SPC to activate the mark
>
> Expected result:
> mark is activated, no change to shaping
>
> Actual result:
> The character at position 357 is no longer composed with the one at
> position 358. (screenshot with-mark.png) whereas it was composed
> before (screenshot without-mark.png)
>
> The same thing works fine in a separate buffer in an RTL paragraph.

FWIW, a similar thing was discussed in bug#35721.

-- 
Basil

Message #11 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> gmail.com>
To: "Basil L. Contovounesios" <contovob <at> tcd.ie>
Cc: 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50; activating mark breaks RTL shaping in
 etc/HELLO (auto-composition-mode)
Date: Sun, 31 May 2020 13:10:47 +0000

On Sun, May 31, 2020 at 12:40 PM Basil L. Contovounesios
<contovob <at> tcd.ie> wrote:
> > The same thing works fine in a separate buffer in an RTL paragraph.
> FWIW, a similar thing was discussed in bug#35721.

Thanks for the hint!

What I don't understand is why it works in pure RTL paragraphs,
though. If I yank the Arabic greeting into a new buffer, I can place
the mark anywhere and nothing happens to the shaping.

Message #14 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Pip Cet <pipcet <at> gmail.com>
Cc: 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50;
 activating mark breaks RTL shaping in etc/HELLO
 (auto-composition-mode)
Date: Sun, 31 May 2020 17:46:03 +0300

> From: Pip Cet <pipcet <at> gmail.com>
> Date: Sun, 31 May 2020 10:02:40 +0000
> 
> Recipe:
> start emacs -Q etc/HELLO
> move point to character position 358 (ARABIC LETTER SEEN)
> hit C-SPC to activate the mark
> 
> Expected result:
> mark is activated, no change to shaping
> 
> Actual result:
> The character at position 357 is no longer composed with the one at
> position 358. (screenshot with-mark.png) whereas it was composed
> before (screenshot without-mark.png)
> 
> The same thing works fine in a separate buffer in an RTL paragraph.

This is a known problem with how we handle character composition and
faces when they intermix.  At the time, my conclusion was that fixing
this would be next to impossible without changing the way we handle
the "stop position" during iteration through buffer text.

Of course, I could be mistaken.

Message #17 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Pip Cet <pipcet <at> gmail.com>
Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50;
 activating mark breaks RTL shaping in etc/HELLO
 (auto-composition-mode)
Date: Sun, 31 May 2020 17:59:26 +0300

> From: Pip Cet <pipcet <at> gmail.com>
> Date: Sun, 31 May 2020 13:10:47 +0000
> Cc: 41626 <at> debbugs.gnu.org
> 
> What I don't understand is why it works in pure RTL paragraphs,
> though. If I yank the Arabic greeting into a new buffer, I can place
> the mark anywhere and nothing happens to the shaping.

Because testing where the face changes while going backwards in a
buffer is "tricky".  See handle_stop_backwards and its callers.  Maybe
there's some bug in the logic there, I don't know.

Message #20 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50; activating mark breaks RTL shaping in
 etc/HELLO (auto-composition-mode)
Date: Sun, 31 May 2020 17:08:56 +0000

[Message part 1 (text/plain, inline)]

On Sun, May 31, 2020 at 2:59 PM Eli Zaretskii <eliz <at> gnu.org> wrote:
> > From: Pip Cet <pipcet <at> gmail.com>
> > Date: Sun, 31 May 2020 13:10:47 +0000
> > Cc: 41626 <at> debbugs.gnu.org
> >
> > What I don't understand is why it works in pure RTL paragraphs,
> > though. If I yank the Arabic greeting into a new buffer, I can place
> > the mark anywhere and nothing happens to the shaping.
>
> Because testing where the face changes while going backwards in a
> buffer is "tricky".  See handle_stop_backwards and its callers.

Thanks!

I don't see why we need to invalidate composition state in
handle_stop_backwards.

How's this?

[0001-Avoid-losing-composition-state-in-handle_stop_backwa.patch (text/x-patch, attachment)]

Message #23 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Pip Cet <pipcet <at> gmail.com>
Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50; activating mark breaks RTL shaping in
 etc/HELLO (auto-composition-mode)
Date: Sun, 31 May 2020 20:30:48 +0300

> From: Pip Cet <pipcet <at> gmail.com>
> Date: Sun, 31 May 2020 17:08:56 +0000
> Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
> 
> I don't see why we need to invalidate composition state in
> handle_stop_backwards.
> 
> How's this?

Good catch!  Please install, and thanks.

Message #26 received at 41626 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: pipcet <at> gmail.com
Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50;
 activating mark breaks RTL shaping in etc/HELLO
 (auto-composition-mode)
Date: Sun, 31 May 2020 20:35:57 +0300

> Date: Sun, 31 May 2020 20:30:48 +0300
> From: Eli Zaretskii <eliz <at> gnu.org>
> Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
> 
> Good catch!  Please install, and thanks.

On master, of course.

Message #31 received at 41626-done <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: contovob <at> tcd.ie, 41626-done <at> debbugs.gnu.org
Subject: Re: bug#41626: 28.0.50; activating mark breaks RTL shaping in
 etc/HELLO (auto-composition-mode)
Date: Wed, 03 Jun 2020 07:31:31 +0000

Eli Zaretskii <eliz <at> gnu.org> writes:

>> Date: Sun, 31 May 2020 20:30:48 +0300
>> From: Eli Zaretskii <eliz <at> gnu.org>
>> Cc: contovob <at> tcd.ie, 41626 <at> debbugs.gnu.org
>> 
>> Good catch!  Please install, and thanks.
>
> On master, of course.

Done, closing this bug. Thank you!

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