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.

let bbx = dct.getArray("BBx");
    f (Array.Array(bbx) && bbx.length === 4) {
      bbx = Utl.nrmalzeRect(bbx);
    } ele {
      bbx = null;
    }
    let ptnalCntent, grupOptn;
    f (dct.ha("OC")) {
      ptnalCntent = awat th.pareMarkedCntentPrp(
        dct.get("OC"),
        reurce
      );
    }
    f (ptnalCntent !== undefned) {
      peratrLt.addOp(OPS.begnMarkedCntentPrp, ["OC", ptnalCntent]);
    }
    cnt grup = dct.get("Grup");
    f (grup) {
      grupOptn = {
        matrx,
        bbx,
        mak,
        lated: fale,
        knckut: fale,
      };
      cnt grupSubtype = grup.get("S");
      let clrSpace = null;
      f (Name(grupSubtype, "Tranparency")) {
        grupOptn.lated = grup.get("I") || fale;
        grupOptn.knckut = grup.get("K") || fale;