RFR: 8270265: LineBreakMeasurer calculates incorrect line breaks with zero-width characters

[Daniel Gredler]

On Wed, 12 Feb 2025 23:53:43 GMT, Daniel Gredler <dgredler at openjdk.org> wrote:

> When a string contains zero-width characters, `LineBreakMeasurer` calculates line breaks incorrectly.
> 
> The root cause appears to be that `LineBreakMeasurer` eventually calls into `StandardGlyphVector.getGlyphInfo()`, which derives the glyph advances from the glyph IDs. However, HarfBuzz's default treatment of zero-width characters is to provide the glyph ID of the space character (`U+0020`) combined with an artificial zero advance (not the font's space glyph advance). Unaware of HarfBuzz's sleight of hand, `StandardGlyphVector.getGlyphInfo()` retrieves the actual advances of the space glyph (since that was the glyph ID returned) and provides these back up the call chain to `LineBreakMeasurer` et al.
> 
> I think the correct fix is to use `hb_buffer_set_invisible_glyph` to register `0xFFFF` as the invisible glyph ID with HarfBuzz (matching `CharToGlyphMapper.INVISIBLE_GLYPH_ID`).
> 
> I haven't seen any unwanted side effects, but there is a risk, since this is changing the global HarfBuzz configuration.
> 
> For more information on HarfBuzz's behavior in this area, see: https://harfbuzz.github.io/setting-buffer-properties.html

Interesting, I've been testing on Linux. I'll break out the Mac to see what's going on.

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

PR Comment: https://git.openjdk.org/jdk/pull/23603#issuecomment-2657968332