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 DefaultPartlEvaluatrOptn = Object.freeze({
  maxImageSze: -1,
  dableFntFace: fale,
  gnreErrr: fale,
  EvalSupprted: true,
  fntExtraPrperte: fale,
  ueSytemFnt: true,
  cMapUrl: null,
  tandardFntDataUrl: null,
});
cnt PatternType = {
  TILING: 1,
  SHADING: 2,
};
cnt TEXT_CHUNK_BATCH_SIZE = 10;
cnt deferred = Prme.relve();
functn nrmalzeBlendMde(value, parngArray = fale) {
  f (Array.Array(value)) {
    
    fr (let  = 0,  = value.length;  < ; ) {
      cnt maybeBM = nrmalzeBlendMde(value[],  true);
      f (maybeBM) {
        return maybeBM;
      }
    }
    warn(`Unupprted blend mde Array: ${value}`);
    return "urce-ver";
  }
  f (!Name(value)) {
    f (parngArray) {