bug#41645: 27.0.91; Combining Grapheme Joiner (#x34f) gui artifacts

[Pip Cet]

David Fussner <dfussner@googlemail.com> writes:
> A couple of data points, in case they're helpful:

Thanks again for testing.

> On 27.0.91 _unpatched_, I see the artifact whenever the font of the
> CGJ is different from that of the glyph before it, no matter which
> script I'm using. When the font of the CGJ and the previous glyph are
> the same, I don't see the artifact, except in Hebrew, where it's still
> present.

Which font are you using for Hebrew text?

> C-u C-x = displays the CGJ on its own, as a separate glyph,
> whenever it's used in Hebrew and also whenever its font doesn't match
> that of the glyph before it. When the font does match, in Latin or
> Greek script, the cursor doesn't stop on the CGJ, and C-u C-x = shows
> it as composed with the previous character.


That sounds as it should be. I'm not sure I understand what you're
seeing in Hebrew text, though: you said you saw the artifact there, but
also that the CGJ is displayed as a separate glyph. Is that corrcet?

> With Pip Cet's second patch, 27.0.91 shows exactly the same behavior
> with C-u C-x =, but the visual artifact never appears, at least in my
> testing, neither in Hebrew nor in the LTR scripts.

So that sounds like an improvement.

While I think we definitely want the patch I sent , it doesn't solve the
real issue: zero-width glyph strings. If we want to allow those, a lot
of the display code has to be changed (we're going to have to figure out
how to show the cursor, for starters); if we don't, that's a change to
the composition-function interface, albeit a minor one.

> Hope this helps.
>
> On Mon, 1 Jun 2020 at 23:37, Pip Cet <pipcet@gmail.com> wrote:
>>
>> On Mon, Jun 1, 2020 at 7:48 PM Pip Cet <pipcet@gmail.com> wrote:
>> > > > Indeed, the composition gstring is a single zero-width glyph.
>> > > See the composition information above: my interpretation of it is that
>> > > the composed glyph is not zero-width.
>> >
>> > ... something is odd here, I agree.
>>
>> I think it's a very odd combination of things:
>> 1. a font which defines an isolated CGJ to have zero width
>> 2. an isolated CGJ appearing in the first place (in this case, because
>> another font does not support CGJ)
>> 3. the fall-back [nil 0 compose-gstring-for-graphic] rule defined for
>> codepoint #x34f
>> 4. compose-gstring-for-graphic attempting to salvage non-spacing
>> characters not following base characters, and producing zero-width
>> lgstrings from zero-width lglyphs
>>
>> Avoiding any of the four will avoid the problem. (1) is something we
>> cannot fix directly. (2) is also something that a user may want. (3)
>> could be dropped, and (4) could be expanded to take care of the
>> zero-width case.
>>
>> However, as long as zero-width gstrings can somehow slip through, I
>> suggest we also apply the patch I sent, assuming it fixes the problem.
>>
>> We might consider simply prohibiting zero-width zero-lbearing
>> zero-rbearing gstrings, the way we prohibit zero-width zero-lbearing
>> zero-rbearing characters in the code I posted.