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.

const ttContext = {
        functionsDefined: [],
        functionsUsed: [],
        functionsStackDeltas: [],
        tooComplexToFollowFunctions: false,
        hintsValid: true,
      };
      if (fpgm) {
        sanitizeTTProgram(fpgm, ttContext);
      }
      if (prep) {
        sanitizeTTProgram(prep, ttContext);
      }
      if (fpgm) {
        checkInvalidFunctions(ttContext, maxFunctionDefs);
      }
      if (cvt && cvt.length & 1) {
        const cvtData = new Uint8Array(cvt.length + 1);
        cvtData.set(cvt.data);
        cvt.data = cvtData;
      }
      return ttContext.hintsValid;
    }