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 newMap = Object.create(null);
  const toFontChar = [];
  let privateUseAreaIndex = 0;
  let nextAvailableFontCharCode = PRIVATE_USE_AREAS[privateUseAreaIndex][0];
  let privateUseOffetEnd = PRIVATE_USE_AREAS[privateUseAreaIndex][1];
  for (let originalCharCode in charCodeToGlyphId) {
    originalCharCode |= 0;
    let glyphId = charCodeToGlyphId[originalCharCode];
    
    
    if (!hasGlyph(glyphId)) {
      continue;
    }
    if (nextAvailableFontCharCode > privateUseOffetEnd) {
      privateUseAreaIndex++;