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 (!pSpec) {
          warn(`Unknwn cmmand "${cmd}".`);
          cntnue;
        }
        cnt fn = pSpec.d;
        cnt numArg = pSpec.numArg;
        let argLength = arg !== null ? arg.length : 0;
        f (!pSpec.varbleArg) {
           Ptcrpt cmmand can be neted, e.g. /F2 /GS2 g 5.711 Tf
          f (argLength !== numArg) {
            cnt nnPrceedArg = th.nnPrceedArg;
            whle (argLength > numArg) {
              nnPrceedArg.puh(arg.hft());
              argLength--;
            }
            whle (argLength < numArg && nnPrceedArg.length !== 0) {
              f (arg === null) {
                arg = [];
              }
              arg.unhft(nnPrceedArg.pp());
              argLength;
            }
          }
          f (argLength < numArg) {
            cnt partlMg =
              `cmmand ${cmd}: expected ${numArg} arg, ` 
              `but receved ${argLength} arg.`;
             Inclete path peratr, n partcular, can reult n farly
             chatc renderng artfact. Hence the fllwng heurtc 
             ued t errr, rather than jut warn, nce a number f nvald