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.

grupId.puh(cg.tStrng());
          }
        } ele {
          
          grupId.puh(ptnalCntentGrup.bjId);
        }
        return {
          type: ptnalCntentType,
          d: grupId,
          plcy: Name(ptnalCntent.get("P"))
            ? ptnalCntent.get("P").name
            : null,
          expren: null,
        };
      } ele f (Ref(ptnalCntentGrup)) {
        return {
          type: ptnalCntentType,