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.

properties.fixedPitch = false;
    if (properties.builtInEncoding) {
      
      
      adjustToUnicode(properties, properties.builtInEncoding);
    }
    
    
    
    let glyphZeroId = 1;
    if (font instanceof CFFFont) {
      glyphZeroId = font.numGlyphs - 1;
    }
    const mapping = font.getGlyphMapping(properties);
    let newMapping = null;
    let newCharCodeToGlyphId = mapping;
    
    
    if (!properties.cssFontInfo) {
      newMapping = adjustMapping(
        mapping,
        font.hasGlyphId.bind(font),
        glyphZeroId
      );
      this.toFontChar = newMapping.toFontChar;
      newCharCodeToGlyphId = newMapping.charCodeToGlyphId;
    }
    const numGlyphs = font.numGlyphs;
    function getCharCodes(charCodeToGlyphId, glyphId) {