RFR: 8353230: Emoji rendering regression after JDK-8208377

[Nikita Gubarkov]

On Thu, 3 Apr 2025 11:23:42 GMT, Daniel Gredler <dgredler at openjdk.org> wrote:

> It looks like this regression actually fits into a longer series of fixes / regressions in this area:
> 
> - [JDK-4517298](https://bugs.openjdk.org/browse/JDK-4517298) fixed metrics for zero-width characters, but broke some ligatures / glyph substitutions
> - [JDK-7017058](https://bugs.openjdk.org/browse/JDK-7017058) fixed the ligatures / glyph substitutions, but broke some zero-width metrics
> - [JDK-8208377](https://bugs.openjdk.org/browse/JDK-8208377) fixed some metrics and rendering for zero-width characters, but broke some ligatures / glyph substitutions
> - Now, with this PR, we aim to fix the ligatures without re-breaking zero-width metrics and display
> 
> We have two different types of use cases pulling `CharToGlyphMapper` in two different directions: the users who need raw, untransformed glyph info, and the users who need normalized / transformed glyph info.
> 
> It looks to me like, in the current code base, the only `CharToGlyphMapper` user which requires raw font data is HarfBuzz (explicitly confirmed with the HarfBuzz team here: https://github.com/harfbuzz/harfbuzz/discussions/5234).
> 
> The regression mechanism at play here is that the HarfBuzz font callbacks are currently providing HarfBuzz with transformed glyph info (e.g. ZWJ -> INVISIBLE_GLYPH_ID), which prevents HarfBuzz from recognizing and applying the correct font GSUB substitutions (which involve ZWJ).
> 
> In order to fix this without (yet again) breaking metrics and display behavior elsewhere, I've added two methods to `CharToGlyphMapper` which provide access to raw glyph info, to be used by the HarfBuzz font callbacks: `charToGlyphRaw(int)` and `charToVariationGlyphRaw(int)`.
> 
> Note two intricacies related to `CompositeGlyphMapper`:
> 1. We need to be careful to only cache raw (untransformed) values, to avoid conflicts between requests for a raw version of a glyph and a transformed version of the same glyph. Another option would have been two separate caches, but I don't think that's necessary.
> 2. Consumers who are using `CompositeGlyphMapper.SLOTMASK` to check glyph slots (e.g. `FontRunIterator` and `CTextPipe`) will "see" invisible glyphs as having come from slot 0. This isn't new, and I think it's OK, but something to be aware of.
> 
> The glyph cache handling in `CCharToGlyphMapper` (for macOS) also requires care to avoid mixing value types.
> 
> Please also note that I'm not sure if the tweak to `sunFont.c` is being tested, since FFM is being used by default for HarfBuzz integration. (Is there a plan to remove...

We had similar emoji-related regressions at JetBrains. Although our font-related code diverged from OpenJDK a bit, porting this patch seems to resolve them too. I am not an OpenJDK reviewer, but LGTM nevertheless.

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PR Comment: https://git.openjdk.org/jdk/pull/24412#issuecomment-2806961451