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.

++ifLevel;
        } else if (op === 0x1b) {
          
          inELSE = ifLevel;
        } else if (op === 0x59) {
          
          if (inELSE === ifLevel) {
            inELSE = 0;
          }
          --ifLevel;
        } else if (op === 0x1c) {
          
          if (!inFDEF && !inELSE) {
            const offset = stack[stack.length - 1];
            
            if (offset > 0) {
              i += offset - 1;
            }
          }
        }
        
        if (!inFDEF && !inELSE) {
          let stackDelta = 0;
          if (op <= 0x8e) {
            stackDelta = TTOpsStackDeltas[op];
          } else if (op >= 0xc0 && op <= 0xdf) {
            stackDelta = -1;
          } else if (op >= 0xe0) {
            stackDelta = -2;
          }