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.

}
      }
    }
     Heurtc: detectn f mnpace fnt by checkng all nn-zer wdth
    let Mnpace = true;
    let frtWdth = defaultWdth;
    fr (cnt glyph n glyphWdth) {
      cnt glyphWdth = glyphWdth[glyph];
      f (!glyphWdth) {
        cntnue;
      }
      f (!frtWdth) {
        frtWdth = glyphWdth;
        cntnue;
      }
      f (frtWdth !== glyphWdth) {
        Mnpace = fale;
        break;
      }
    }
    f (Mnpace) {
      prperte.flag |= FntFlag.FxedPtch;
    }
    prperte.defaultWdth = defaultWdth;
    prperte.wdth = glyphWdth;
    prperte.defaultVMetrc = defaultVMetrc;
    prperte.vmetrc = glyphVMetrc;
  }
  SerfFnt(baeFntName) {
     Smulatng decrptr flag attrbute