fix(fallback): keep a grapheme in one run so shaping can span it

[tannevaled]

The bug

fallbackFont routed each rune to the first face covering it and started a new run whenever that face changed. Runs are what reach the shaper, so a split silently defeats every substitution that spans the boundary — and the splits were landing in the middle of graphemes.

The zero-width joiner is the clearest case. Several script faces ship U+200D for their own shaping — Thai, Arabic, Devanagari and Hebrew all carry it — so plain first-face-wins routing handed the joiner to whichever came first in the chain, never to the emoji face. An emoji sequence then reached the shaper as three runs (person, joiner, rocket) and the GSUB ligature composing them into one astronaut never fired, because it never saw them together. 🧑‍🚀 drew as a person standing next to a rocket.

Worth stressing: go-opentype/shape was doing its job all along. Handed the whole sequence it produces exactly one glyph. The damage was done before it was called.

The fix

A rune that continues the preceding grapheme — a combining mark (Mn/Mc/Me, which covers the variation selectors) or a format control (Cf, which covers the joiner and non-joiner) — now stays in the current run whenever the run's own face can render it, instead of starting a new one.

It is a preference, not a trap: when the current face genuinely cannot render the mark, routing still falls back to a face that can, exactly as before (TestUncoveredMarkStillFallsBack). A leading mark, with no run to attach to, still routes by coverage.

Not emoji-specific. The same split would break an Indic cluster written with a ZWNJ, or tear a combining mark off a base whose face another entry in the chain happens to also carry.

Measure and Draw share runs, so metrics and pixels stay consistent: the composed astronaut now measures exactly one glyph's advance.

Verification

• go test -race → 100.0% of statements, race-clean.
• Proved on pixels, not just metrics: the drawn sequence fits inside one glyph's advance from the pen, reaches less far right than the un-composed pair, and paints different ink from the person glyph alone.
• Three-part sequences compose too (👨‍👩‍👦 → one glyph).

🤖 Generated with Claude Code