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 tag = file.getString(4);
      const checksum = file.getInt32() >>> 0;
      const offset = file.getInt32() >>> 0;
      const length = file.getInt32() >>> 0;
      
      const previousPosition = file.pos;
      file.pos = file.start ? file.start : 0;
      file.skip(offset);
      const data = file.getBytes(length);
      file.pos = previousPosition;
      if (tag === "head") {
        
        data[8] = data[9] = data[10] = data[11] = 0;
        data[17] |= 0x20; 
      }
      return {
        tag,
        checksum,
        length,
        offset,
        data,
      };
    }
    function readOpenTypeHeader(ttf) {