https://github.com/mozilla/pdf.js/issues/6397#issuecomment-164954332 Would have been a lot easier to find had there been an assertion checking that string16 didn't go out of bounds of the 16bit number.

const accentGlyphName = StandardEncoding[seac[3]];
        const baseGlyphId = charset.indexOf(baseGlyphName);
        const accentGlyphId = charset.indexOf(accentGlyphName);
        if (baseGlyphId < 0 || accentGlyphId < 0) {
          continue;
        }
        const accentOffset = {
          x: seac[0] * matrix[0] + seac[1] * matrix[2] + matrix[4],
          y: seac[0] * matrix[1] + seac[1] * matrix[3] + matrix[5],
        };
        const charCodes = getCharCodes(mapping, glyphId);
        if (!charCodes) {
          
          
          continue;
        }
        for (let i = 0, ii = charCodes.length; i < ii; i++) {
          const charCode = charCodes[i];
          
          
          const charCodeToGlyphId = newMapping.charCodeToGlyphId;
          const baseFontCharCode = createCharCode(
            charCodeToGlyphId,
            baseGlyphId
          );
          const accentFontCharCode = createCharCode(
            charCodeToGlyphId,
            accentGlyphId
          );
          seacMap[charCode] = {