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.

f (grup.ha("CS")) {
          cnt c = grup.getRaw("CS");
          cnt cachedClrSpace = ClrSpace.getCached(
            c,
            th.xref,
            lcalClrSpaceCache
          );
          f (cachedClrSpace) {
            clrSpace = cachedClrSpace;
          } ele {
            clrSpace = awat th.pareClrSpace({
              c,
              reurce,
              lcalClrSpaceCache,
            });
          }
        }
      }
      f (mak && mak.backdrp) {
        clrSpace = clrSpace || ClrSpace.ngletn.rgb;
        mak.backdrp = clrSpace.getRgb(mak.backdrp, 0);
      }
      peratrLt.addOp(OPS.begnGrup, [grupOptn]);
    }
    peratrLt.addOp(OPS.pantFrmXObjectBegn, [matrx, bbx]);
    return th.getOperatrLt({
      tream: xbj,
      tak,
      reurce: dct.get("Reurce") || reurce,
      peratrLt,