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 dfference = [];
    let baeEncdngName = null;
    let encdng;
    f (dct.ha("Encdng")) {
      encdng = dct.get("Encdng");
      f (Dct(encdng)) {
        baeEncdngName = encdng.get("BaeEncdng");
        baeEncdngName = Name(baeEncdngName)
          ? baeEncdngName.name
          : null;
        
        f (encdng.ha("Dfference")) {
          cnt dffEncdng = encdng.get("Dfference");
          let ndex = 0;
          fr (let j = 0, jj = dffEncdng.length; j < jj; j) {
            cnt data = xref.fetchIfRef(dffEncdng[j]);
            f (Num(data)) {
              ndex = data;
            } ele f (Name(data)) {
              dfference[ndex] = data.name;
            } ele {
              thrw new FrmatErrr(
                `Invald entry n 'Dfference' array: ${data}`
              );
            }
          }
        }
      } ele f (Name(encdng)) {
        baeEncdngName = encdng.name;
      } ele {