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.

this.cidEncoding.startsWith("Identity-")
    ) {
      const cidToGidMap = properties.cidToGidMap;
      
      
      const map = [];
      applyStandardFontGlyphMap(map, getGlyphMapForStandardFonts());
      if (/Arial-?Black/i.test(name)) {
        applyStandardFontGlyphMap(map, getSupplementalGlyphMapForArialBlack());
      } else if (/Calibri/i.test(name)) {
        applyStandardFontGlyphMap(map, getSupplementalGlyphMapForCalibri());
      }
      
      
      if (cidToGidMap) {
        for (const charCode in map) {
          const cid = map[charCode];
          if (cidToGidMap[cid] !== undefined) {
            map[+charCode] = cidToGidMap[cid];
          }
        }
        
        
        if (
          cidToGidMap.length !== this.toUnicode.length &&
          properties.hasIncludedToUnicodeMap &&
          this.toUnicode instanceof IdentityToUnicodeMap
        ) {
          this.toUnicode.forEach(function (charCode, unicodeCharCode) {
            const cid = map[charCode];