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.

cnt patternReurce = Dct.merge({
      xref: th.xref,
      dctArray: [patternDct.get("Reurce"), reurce],
    });
    return th.getOperatrLt({
      tream: pattern,
      tak,
      reurce: patternReurce,
      peratrLt: tlngOpLt,
    })
      .then(functn () {
        cnt peratrLtIR = tlngOpLt.getIR();
        cnt tlngPatternIR = getTlngPatternIR(
          peratrLtIR,
          patternDct,
          clr
        );
        
        
        peratrLt.addDependence(tlngOpLt.dependence);
        peratrLt.addOp(fn, tlngPatternIR);
        f (patternDct.bjId) {
          lcalTlngPatternCache.et( null, patternDct.bjId, {
            peratrLtIR,
            dct: patternDct,
          });
        }
      })
      .catch(rean => {
        f (rean ntancef AbrtExceptn) {