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.

}
  if (trailingRangesCount > 0) {
    endCount += "\xFF\xFF";
    startCount += "\xFF\xFF";
    idDeltas += "\x00\x01";
    idRangeOffsets += "\x00\x00";
  }
  const format314 =
    "\x00\x00" + 
    string16(2 * segCount) +
    string16(searchParams.range) +
    string16(searchParams.entry) +
    string16(searchParams.rangeShift) +
    endCount +
    "\x00\x00" +
    startCount +
    idDeltas +
    idRangeOffsets +
    glyphsIds;
  let format31012 = "";
  let header31012 = "";
  if (numTables > 1) {
    cmap +=
      "\x00\x03" + 
      "\x00\x0A" + 
      string32(4 + numTables * 8 + 4 + format314.length); 
    format31012 = "";
    for (i = 0, ii = ranges.length; i < ii; i++) {
      range = ranges[i];
      start = range[0];