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 (
        properties.hasEncoding &&
        !this.isSymbolicFont &&
        ((cmapPlatformId === 3 && cmapEncodingId === 1) ||
          (cmapPlatformId === 1 && cmapEncodingId === 0))
      ) {
        const glyphsUnicodeMap = getGlyphsUnicode();
        for (let charCode = 0; charCode < 256; charCode++) {
          let glyphName;
          if (this.differences[charCode] !== undefined) {
            glyphName = this.differences[charCode];
          } else if (baseEncoding.length && baseEncoding[charCode] !== "") {
            glyphName = baseEncoding[charCode];
          } else {
            glyphName = StandardEncoding[charCode];
          }
          if (!glyphName) {
            continue;
          }
          
          const standardGlyphName = recoverGlyphName(
            glyphName,
            glyphsUnicodeMap
          );
          let unicodeOrCharCode;
          if (cmapPlatformId === 3 && cmapEncodingId === 1) {
            unicodeOrCharCode = glyphsUnicodeMap[standardGlyphName];
          } else if (cmapPlatformId === 1 && cmapEncodingId === 0) {
            
            unicodeOrCharCode = MacRomanEncoding.indexOf(standardGlyphName);