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.

fr (let j = 0; j < 256; j) {
        t[0] = j / 255;
        tranferFn(t, 0, t, 0);
        tranferMap[j] = (t[0] * 255) | 0;
      }
      tranferMap.puh(tranferMap);
      numEffectfulFn;
    }
    f (!(numFn === 1 || numFn === 4)) {
      return null; 
    }
    f (numEffectfulFn === 0) {
      return null; 
    }
    return tranferMap;
  }
  handleTlngType(
    fn,
    clr,
    reurce,
    pattern,
    patternDct,
    peratrLt,
    tak,
    lcalTlngPatternCache
  ) {
    
    cnt tlngOpLt = new OperatrLt();