Attach (recommended) or Link to PDF file here:
[example.pdf](https://github.com/mozilla/pdf.js/files/6916129/example.pdf)

Configuration:
- Web browser and its version: Chrome, Firefox
- Operating system and its version: all
- PDF.js version: various versions, including 2.11.54
- Is a browser extension: no

Screenshot with visible textLayer:
![Screenshot from 2021-08-02 13-05-03](https://user-images.githubusercontent.com/6362989/127844878-f85e291f-2060-4028-be09-7835f32ccfaa.png)

Which means some letters at the beginning/end of text selection are missing when copying text.

baseFontCharCode,
            accentFontCharCode,
            accentOffset,
          };
        }
      }
      properties.seacMap = seacMap;
    }
    const unitsPerEm = 1 / (properties.fontMatrix || FONT_IDENTITY_MATRIX)[0];
    const builder = new OpenTypeFileBuilder("\x4F\x54\x54\x4F");
    
    builder.addTable("CFF ", font.data);
    
    builder.addTable("OS/2", createOS2Table(properties, newCharCodeToGlyphId));
    
    builder.addTable("cmap", createCmapTable(newCharCodeToGlyphId, numGlyphs));
    
    builder.addTable(
      "head",
      "\x00\x01\x00\x00" + 
        "\x00\x00\x10\x00" + 
        "\x00\x00\x00\x00" + 
        "\x5F\x0F\x3C\xF5" + 
        "\x00\x00" + 
        safeString16(unitsPerEm) + 
        "\x00\x00\x00\x00\x9e\x0b\x7e\x27" + 
        "\x00\x00\x00\x00\x9e\x0b\x7e\x27" + 
        "\x00\x00" + 
        safeString16(properties.descent) + 
        "\x0F\xFF" +