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.

data[offset + 2] = (value >> 8) & 0xff;
          data[offset + 3] = value & 0xff;
        };
      } else {
        itemSize = 2;
        itemDecode = function fontItemDecode(data, offset) {
          return (data[offset] << 9) | (data[offset + 1] << 1);
        };
        itemEncode = function fontItemEncode(data, offset, value) {
          data[offset] = (value >> 9) & 0xff;
          data[offset + 1] = (value >> 1) & 0xff;
        };
      }
      
      const numGlyphsOut = dupFirstEntry ? numGlyphs + 1 : numGlyphs;
      const locaDataSize = itemSize * (1 + numGlyphsOut);
      
      const locaData = new Uint8Array(locaDataSize);
      locaData.set(loca.data.subarray(0, locaDataSize));
      loca.data = locaData;
      
      const oldGlyfData = glyf.data;
      const oldGlyfDataLength = oldGlyfData.length;
      const newGlyfData = new Uint8Array(oldGlyfDataLength);
      
      
      
      
      
      let i, j;