Treat Indic conjunct clusters as a single grapheme (UAX #29 GB9c)

[greymoth-jp]

Description

getCharacterDistance implements the UAX #29 extended grapheme cluster rules (GB6 through GB13) but not GB9c, which was added in Unicode 15.1. GB9c keeps an Indic conjunct together:

Consonant [Extend Linker]* Linker [Extend Linker]* × Consonant

Without it, a Consonant + virama + Consonant sequence breaks right after the virama. Since getCharacterDistance and getWordDistance back Editor.positions (and the Editor.before / Editor.after built on it), moving the cursor by character through scripts like Devanagari or Bengali stops in the middle of a conjunct that the reader sees as one glyph.

क्त (U+0915 U+094D U+0924) is one grapheme, but getCharacterDistance('क्त') returned 2 (just क्) instead of 3.

Issue

No existing issue.

Example

// before
getCharacterDistance('क्त') // 2  ->  क् | त
// after
getCharacterDistance('क्त') // 3  ->  क्त

This also covers a conjunct with an intervening ZWJ (क्‍त) and chained conjuncts (क्त्य), and still breaks between two consonants that have no linker between them (क | त).

Context

The implementation follows the existing deferred-rule pattern. Three regex classes for the Indic_Conjunct_Break property values (Consonant, Extend, Linker) are taken from DerivedCoreProperties.txt, in the same \uXXXX style as the existing tables. GB9c is added to NonBoundaryPairs as [InCBLinker | InCBExtend, InCBConsonant], with a deferred look-behind (endsWithConjunctLinker) that confirms the full left context before suppressing the break, the same way the GB11 emoji-ZWJ case is handled.

I checked the change against the official Unicode GraphemeBreakTest.txt (17.0.0): every GB9c line now passes in both LTR and RTL directions, and no previously passing line changed. The only failures left in that file are CR/LF control-character sequences (GB3 to GB5), which are unrelated to this change. Regression cases for the conjunct strings were added to test/utils/string.ts.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9763fd2

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[dylans]

Prettier is complaining so please clean up the PR, then this should be ok. There's a bit of extra comments that feel unnecessary that could be cleaned up.

[greymoth-jp]

Ran prettier and trimmed the comments down to the GB9c note. Thanks for the review.