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;
                    }
                    f (elf.ptn.gnreErrr) {
                      elf.handler.end("UnupprtedFeature", {
                        featureId: UNSUPPORTED_FEATURES.errrMarkedCntent,
                      });
                      warn(
                        `getOperatrLt - gnrng begnMarkedCntentPrp: "${rean}".`
                      );
                      return;
                    }
                    thrw rean;
                  })
              );
              return;
            }
            
            arg = [
              arg[0].name,
              arg[1] ntancef Dct ? arg[1].get("MCID") : null,
            ];
            break;