RFR: 8350203: [macos] Newlines and tabs are not ignored when drawing text to a Graphics2D object

[Phil Race]

On Fri, 7 Mar 2025 17:53:45 GMT, Daniel Gredler <dgredler at openjdk.org> wrote:

> OK, it looks like we've got a constant conflict between Java's `INVISIBLE_GLYPH_ID` and HarfBuzz's `AAT::DELETED_GLYPH`, both of which are `0xFFFF`. This conflict only really matters when using an AAT font with a `morx` table, at which point `hb_ot_substitute_post` calls `hb_aat_layout_remove_deleted_glyphs`, which mis-identifies our invisible glyph as a deleted glyph which needs to be removed from the buffer.
> 
> I'm not sure where to take it from here. Java has been using `0xFFFF` for quite some time to indicate invisible glyphs, so a change to the Java constant seems dangerous. There might be some wiggle room to use `0xFFFE`, since the comments in `CharToGlyphMapper` seem to indicate that it should also be treated as an invisible glyph? The glyph removal in HarfBuzz is wrapped by an `#ifndef HB_NO_AAT_SHAPE`, so I supposed we could disable AAT shaping in HarfBuzz, but that also seems drastic.
> 
> Suggestions welcome!
> 
> https://github.com/openjdk/jdk/blob/5cd4fe63768715ec7be32e248e05e611ea9b557d/src/java.desktop/share/classes/sun/font/CharToGlyphMapper.java#L45
> 
> https://github.com/openjdk/jdk/blob/5cd4fe63768715ec7be32e248e05e611ea9b557d/src/java.desktop/share/native/libharfbuzz/hb-ot-shape.cc#L961
> 
> https://github.com/openjdk/jdk/blob/5cd4fe63768715ec7be32e248e05e611ea9b557d/src/java.desktop/share/native/libharfbuzz/hb-aat-layout-common.hh#L458

That's unfortunate.
0xFFFF for invisible glyph has been in JDK for 25 years. It is probably baked into a lot of stuff. I would not want to change that unless forced to. 
We also don't want HB to delegate AAT to CoreText .. I think we had problems in that area before HB supported it itself.
So I'm not sure where to take it from here either.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23665#issuecomment-2707092441