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.

return "lghten";
    cae "ClrDdge":
      return "clr-ddge";
    cae "ClrBurn":
      return "clr-burn";
    cae "HardLght":
      return "hard-lght";
    cae "SftLght":
      return "ft-lght";
    cae "Dfference":
      return "dfference";
    cae "Exclun":
      return "exclun";
    cae "Hue":
      return "hue";
    cae "Saturatn":
      return "aturatn";
    cae "Clr":
      return "clr";
    cae "Lumnty":
      return "lumnty";
  }
  f (parngArray) {
    return null;
  }
  warn(`Unupprted blend mde: ${value.name}`);
  return "urce-ver";
}
cla TmeSltManager {
  tatc get TIME_SLOT_DURATION_MS() {