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.

"\x01\xDF" + 
    "\x00\x31" + 
    "\x01\x02" + 
    "\x00\x00" + 
    "\x00\x00\x06" +
    String.fromCharCode(properties.fixedPitch ? 0x09 : 0x00) +
    "\x00\x00\x00\x00\x00\x00" + 
    string32(ulUnicodeRange1) + 
    string32(ulUnicodeRange2) + 
    string32(ulUnicodeRange3) + 
    string32(ulUnicodeRange4) + 
    "\x2A\x32\x31\x2A" + 
    string16(properties.italicAngle ? 1 : 0) + 
    string16(firstCharIndex || properties.firstChar) + 
    string16(lastCharIndex || properties.lastChar) + 
    string16(typoAscent) + 
    string16(typoDescent) + 
    "\x00\x64" + 
    string16(winAscent) + 
    string16(winDescent) + 
    "\x00\x00\x00\x00" + 
    "\x00\x00\x00\x00" + 
    string16(properties.xHeight) + 
    string16(properties.capHeight) + 
    string16(0) + 
    string16(firstCharIndex || properties.firstChar) + 
    "\x00\x03"
  ); 
}
function createPostTable(properties) {