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 (!prperte.cte ) {
      return new TUncdeMap(th._leFntTUncde(prperte));
    }
    
    
     d
     A
    f (
      prperte.cte &&
      ((prperte.cMap.bultInCMap &&
        !(prperte.cMap ntancef IdenttyCMap)) ||
        (prperte.cdSytemInf.regtry === "Adbe" &&
          (prperte.cdSytemInf.rderng === "GB1" ||
            prperte.cdSytemInf.rderng === "CNS1" ||
            prperte.cdSytemInf.rderng === "Japan1" ||
            prperte.cdSytemInf.rderng === "Krea1")))
    ) {
       T
       a
       t
       b) 
       by the fnt’ CMap (fr exale, Adbe and Japan1) frm t
       CIDSytemInf dctnary.
      cnt { regtry, rderng } = prperte.cdSytemInf;
       c) Cntruct a ecnd CMap name by cncatenatng the regtry and
       rderng btaned n tep (b) n the frmat regtry–rderng–UCS2
       (fr exale, Adbe–Japan1–UCS2).
      cnt uc2CMapName = Name.get(`${regtry}-${rderng}-UCS2`);
       d) Obtan the CMap wth the name cntructed n tep (c) (avalable