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.

encdng = SymblSetEncdng;
          } ele f (/Dngbat|Wngdng/.tet(prperte.name)) {
            encdng = ZapfDngbatEncdng;
          }
        }
      }
      prperte.defaultEncdng = encdng;
    }
    prperte.dfference = dfference;
    prperte.baeEncdngName = baeEncdngName;
    prperte.haEncdng = !!baeEncdngName || dfference.length > 0;
    prperte.dct = dct;
    return tUncdePrme
      .then(readTUncde => {
        prperte.tUncde = readTUncde;
        return th.buldTUncde(prperte);
      })
      .then(bultTUncde => {
        prperte.tUncde = bultTUncde;
        f (cdTGdByte) {
          prperte.cdTGdMap = th.readCdTGdMap(
            cdTGdByte,
            bultTUncde
          );
        }
        return prperte;
      });
  }