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

[Pip Cet <pipcet@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Pip Cet <pipcet@gmail.com>
>> Cc: dfussner@googlemail.com,  41645@debbugs.gnu.org
>> Date: Tue, 02 Jun 2020 19:21:31 +0000
>> 
>> >> xdisp.c:
>> >> 30008 if (get_char_glyph_code (it->char_to_display, font,
>> >> &char2b))
>> >> 30009            {
>> >> 30010              pcm = get_per_char_metric (font, &char2b);
>> >> 30011              if (pcm->width == 0
>> >> 30012              && pcm->rbearing == 0 && pcm->lbearing == 0)
>> >> 30013            pcm = NULL;
>> >> 30014            }
>> >> 
>> >
>> > ...with this.  I think you are right, and we should do the same with
>> > zero-width LGLYPH_STRING, forcing it->glyph_not_available_p to
>> > non-zero, and then doing something like this in
>> > fill_gstring_glyph_string:
>> >
>> >   if (s->font == NULL || glyph_not_available_p)
>> >     {
>> >       s->font_not_found_p = true;
>> >       s->font = FRAME_FONT (s->f);
>> >     }
>> >
>> > similar to what fill_glyph_string does.  WDYT?
>> 
>> I agree; the more I think about it, the more dangerous zero-sized
>> characters seem to me.
>> 
>> And almost all of my concerns apply to characters with zero x advance,
>> no matter whether they have lbearing or rbearing > 0.
>
> AFAIU, the advance metric is what we call "pixel width", and if so,
> the above snippet from xdisp.c already tests it.  Right?

Some code after it does, yes.

>> Maybe, for master, we should reject those as well? I was going to say
>> "or force their width to be at least a single pixel", but I'm not even
>> sure that's sufficient on hidpi screens...
>> 
>> So, in summary, I'd like to do the following:
>> 
>> 1. abort if we ever find ourselves drawing a zero-width cursor
>> 2. reject lgstrings of zero width
>> 3. reject all glyphs of zero width outside of compositions
>> 4. allow users to specify a minimum width, perhaps relative to the font
>> size, so they can always see their cursor
>> 
>> ...which would be quite a different patch.
>
> I'm not sure I follow.  What do you mean by "reject"?  I thought the
> code which ignores the metric and sets the font_not_found_p flag when
> we get a zero-width glyph is a kind of "rejection".

I mean "treat the glyph as non-existent". Currently, for (3), glyphs
with lbearing but no pixel width are treated as valid and expanded to
cover a single pixel, which is all but invisible on my screen.

> I like the idea
> of doing that in the case of compositions because that is consistent
> with what we do when we extract the metrics directly from the font.

I do too.

> Are you saying that what we do with simple characters in this case is
> not good enough?

I'm saying it's not good enough for non-spacing characters that actually
do print something: they're displayed as single-pixel-wide glyphs, which
I think is insufficient to let users be aware of them.

> If you disable auto-composition-mode, and use the
> fonts which shows CGJ as zero-width glyph, do you still see display
> artifacts?

No.

> If not, what do you see and why is this kind of
> 'rejection" not enough?

CGJ is displayed as a black box as wide as a space, which is perfectly
fine. It's U+301 that's not.