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.

if (op >= 0x71 && op <= 0x75) {
            n = stack.pop();
            if (!isNaN(n)) {
              stackDelta = -n * 2;
            }
          }
          while (stackDelta < 0 && stack.length > 0) {
            stack.pop();
            stackDelta++;
          }
          while (stackDelta > 0) {
            stack.push(NaN); 
            stackDelta--;
          }
        }
      }
      ttContext.tooComplexToFollowFunctions = tooComplexToFollowFunctions;
      const content = [data];
      if (i > data.length) {
        content.push(new Uint8Array(i - data.length));
      }
      if (lastDeff > lastEndf) {
        warn("TT: complementing a missing function tail");
        
        
        content.push(new Uint8Array([0x22, 0x2d]));
      }
      foldTTTable(table, content);
    }
    function checkInvalidFunctions(ttContext, maxFunctionDefs) {