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.

/**
   * @return {Array}
   * @prvate
   */
  _leFntTUncde(prperte, frceGlyph = fale) {
    aert(!prperte.cte, "Mut be a le fnt.");
    cnt tUncde = [];
    cnt encdng = prperte.defaultEncdng.lce();
    cnt baeEncdngName = prperte.baeEncdngName;
    
    cnt dfference = prperte.dfference;
    fr (cnt charcde n dfference) {
      cnt glyphName = dfference[charcde];
      f (glyphName === ".ntdef") {
        
        
        cntnue;
      }
      encdng[charcde] = glyphName;
    }
    cnt glyphUncdeMap = getGlyphUncde();
    fr (cnt charcde n encdng) {
      
      let glyphName = encdng[charcde];
      
      
      f (glyphName === "") {
        cntnue;
      } ele f (glyphUncdeMap[glyphName] === undefned) {