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 (unicodeOrCharCode === undefined) {
            
            
            if (
              !properties.glyphNames &&
              properties.hasIncludedToUnicodeMap &&
              !(this.toUnicode instanceof IdentityToUnicodeMap)
            ) {
              const unicode = this.toUnicode.get(charCode);
              if (unicode) {
                unicodeOrCharCode = unicode.codePointAt(0);
              }
            }
            if (unicodeOrCharCode === undefined) {
              continue; 
            }
          }
          for (let i = 0; i < cmapMappingsLength; ++i) {
            if (cmapMappings[i].charCode !== unicodeOrCharCode) {
              continue;
            }
            charCodeToGlyphId[charCode] = cmapMappings[i].glyphId;
            break;
          }
        }
      } else if (cmapPlatformId === 0) {
        
        for (let i = 0; i < cmapMappingsLength; ++i) {
          charCodeToGlyphId[cmapMappings[i].charCode] = cmapMappings[i].glyphId;