bug#14461: 24.3.50; bad display for 'space' + (U+0336) unicode combination

[Stephen Berman <stephen.berman@gmx.net>]

On Sat, 17 Aug 2019 18:09:17 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: handa@gnu.org,  cedric.chepied@gmail.com,  14461@debbugs.gnu.org,
>>   larsi@gnus.org
>> Date: Sat, 17 Aug 2019 16:40:44 +0200
>>
>> >                                                   If so, I think this
>> > just means that the default font you use doesn't support these
>> > combining accents, because on my system I see a single grapheme
>> > cluster in both of the above cases, when I select a suitable font.
>>
>> My default font is DejaVu Sans Mono, but it seems there's something else
>> at play here: in contrast to 'aU+0301U+0302', I do see the sequence
>> 'bU+0301U+0302' as a single grapheme cluster.  Maybe the difference is
>> because there is a glyph for 'a' with an acute accent and it doesn't
>> support further combining.  (But I have no idea if that makes sense.)
>> Here's what describe-char shows on both:
>
> That says you have a single grapheme cluster in both cases.  Does the
> cursor include all of the characters in both cases?

Visually, in the case of the 'a' sequence, the cursor does not cover the
circumflex, but...

>                                                      IOW, can you move
> with C-f between these 3 characters, or do they behave as a single
> character cell in both cases?

Yes, with the 'a' sequence, when I type C-f, the cursor now appears over
the circumflex, but describe-char says the character at that position is
C-j, and typing C-f indeed advances point to the next line.

>                                If the latter, then the composition was
> done in both cases, and you simply should find a better font if you
> want these displayed more nicely.

That indeed appears to be the case: when I change the font to DejaVu
Sans (i.e. not the monospace version), then the 'a' sequence is
displayed like the 'b' sequence, with both combining characters over the
alphabetic character.  This seems like a bug in the monospace font, but
it also seems unlikely such a bug wouldn't have been noticed and fixed
long ago, so I suspect there must be some other reason for the
difference.

Steve Berman