Chaining substitution throws on nested Single (type 1, format 1) lookups

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Summary

font.stringToGlyphs() throws for fonts whose GSUB uses a Chaining Context Substitution (lookup type 6), format 3 whose SubstLookupRecord points to a Single Substitution (lookup type 1), format 1 lookup.

Error: Substitution type 11 is not supported in chaining substitution

chainingSubstitutionFormat3 (in src/features/featureQuery.mjs) only handles the nested substitution types "12" (Single, format 2) and "21" (Multiple, format 1), and throws for anything else. A nested Single/format-1 lookup resolves to "11", which is not handled.

Current Behavior

stringToGlyphs() throws Error: Substitution type 11 is not supported in chaining substitution and shaping aborts for the whole run.

Expected Behavior

The nested Single (format 1) substitution should be applied (same as Single format 2), and stringToGlyphs() should return the substituted glyphs without throwing.

Steps to Reproduce

Any font with a GSUB ChainContextSubst (type 6) format-3 subtable whose SubstLookupRecord references a Single-format-1 lookup triggers this. A concrete open-source example is Noto Sans Thai Looped (Google Fonts, OFL): the ccmp feature substitutes ฬ (U+0E2C) to a short-tail variant when an above mark follows, via a chaining context whose nested lookup is a Single/format-1 substitution.

import opentype from "opentype.js";
import fs from "fs";

const buf = fs.readFileSync("NotoSansThaiLooped-Regular.otf");
const font = opentype.parse(buf.buffer.slice(buf.byteOffset, buf.byteOffset + buf.byteLength));

// U+0E2C (ฬ) + U+0E34 (above vowel ◌ิ)
font.stringToGlyphs("ฬิ"); // throws: Substitution type 11 is not supported in chaining substitution

Related

• Fix OpenType Context Substitution Format 3 and decomposition type handling for Arabic/emoji shaping #824 added nested type "21" handling for Arabic/emoji, but not "11".
• Add support for chaining and context substitution tables #816 adds outer ChainContext format 1/2 and Context format 2 — a different gap (outer subtable format, not the nested lookup type), so it does not cover this case.

Your Environment

• opentype.js version: 2.0.0 (latest on npm)
• Environment: Node.js (also reproducible in browsers)