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.

source,
      sourceStart,
      sourceEnd,
      dest,
      destStart,
      hintsValid
    ) {
      const glyphProfile = {
        length: 0,
        sizeOfInstructions: 0,
      };
      if (sourceEnd - sourceStart <= 12) {
        
        return glyphProfile;
      }
      const glyf = source.subarray(sourceStart, sourceEnd);
      let contoursCount = signedInt16(glyf[0], glyf[1]);
      if (contoursCount < 0) {
        
        contoursCount = -1;
        writeSignedInt16(glyf, 0, contoursCount);
        
        dest.set(glyf, destStart);
        glyphProfile.length = glyf.length;
        return glyphProfile;
      }
      let i,