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

[Alisen Chung]

On Fri, 14 Feb 2025 00:04:29 GMT, Phil Race <prr 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
>
> Early days but the test fails on macOS
> Exception in thread "main" java.lang.RuntimeException: nextOffset 1 for char 00ad using font Dialog: 2 != 1
> 	at FormatCharAdvanceTest.assertEqual(FormatCharAdvanceTest.java:289)
> 	at FormatCharAdvanceTest.testChar(FormatCharAdvanceTest.java:282)
> 	at FormatCharAdvanceTest.testChars(FormatCharAdvanceTest.java:165)
> 	at FormatCharAdvanceTest.main(FormatCharAdvanceTest.java:154)

> @prrace Two findings here:
> 
> First, it looks like macOS needs an extra pixel of wiggle room in the max string width that we measure; I've given it two pixels, just to be extra sure that the test is stable.

Should you add only these pixels when running the test on macos? Or do these pixels not matter on other platforms?

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PR Comment: https://git.openjdk.org/jdk/pull/23603#issuecomment-2675394575