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.

ttContext.hintsValid = false;
                  return;
                }
                stack.length = newStackLength;
              } else if (
                funcId in ttContext.functionsDefined &&
                !functionsCalled.includes(funcId)
              ) {
                callstack.push({ data, i, stackTop: stack.length - 1 });
                functionsCalled.push(funcId);
                pc = ttContext.functionsDefined[funcId];
                if (!pc) {
                  warn("TT: CALL non-existent function");
                  ttContext.hintsValid = false;
                  return;
                }
                data = pc.data;
                i = pc.i;
              }
            }
          }
        } else if (op === 0x2c && !tooComplexToFollowFunctions) {
          
          if (inFDEF || inELSE) {
            warn("TT: nested FDEFs not allowed");
            tooComplexToFollowFunctions = true;
          }
          inFDEF = true;
          
          lastDeff = i;