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.defaultEncoding,
        getGlyphsUnicode(),
        this.differences
      );
      if (
        type === "CIDFontType2" &&
        !this.cidEncoding.startsWith("Identity-") &&
        !(this.toUnicode instanceof IdentityToUnicodeMap)
      ) {
        this.toUnicode.forEach(function (charCode, unicodeCharCode) {
          map[+charCode] = unicodeCharCode;
        });
      }
      this.toFontChar = map;
    } else {
      const glyphsUnicodeMap = getGlyphsUnicode();
      const map = [];
      this.toUnicode.forEach((charCode, unicodeCharCode) => {
        if (!this.composite) {
          const glyphName =
            this.differences[charCode] || this.defaultEncoding[charCode];
          const unicode = getUnicodeForGlyph(glyphName, glyphsUnicodeMap);
          if (unicode !== -1) {
            unicodeCharCode = unicode;
          }
        }
        map[+charCode] = unicodeCharCode;
      });