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.

safeString16(properties.ascent) + 
        string16(properties.italicAngle ? 2 : 0) + 
        "\x00\x11" + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00"
    ); 
    
    builder.addTable(
      "hhea",
      "\x00\x01\x00\x00" + 
        safeString16(properties.ascent) + 
        safeString16(properties.descent) + 
        "\x00\x00" + 
        "\xFF\xFF" + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00" + 
        safeString16(properties.capHeight) + 
        safeString16(Math.tan(properties.italicAngle) * properties.xHeight) + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00" + 
        "\x00\x00" + 
        string16(numGlyphs)
    ); 
    
    builder.addTable(