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.

font.skip(28);
      let glyphNames;
      let valid = true;
      let i;
      switch (version) {
        case 0x00010000:
          glyphNames = MacStandardGlyphOrdering;
          break;
        case 0x00020000:
          const numGlyphs = font.getUint16();
          if (numGlyphs !== maxpNumGlyphs) {
            valid = false;
            break;
          }
          const glyphNameIndexes = [];
          for (i = 0; i < numGlyphs; ++i) {
            const index = font.getUint16();
            if (index >= 32768) {
              valid = false;
              break;
            }
            glyphNameIndexes.push(index);
          }
          if (!valid) {
            break;
          }
          const customNames = [],
            strBuf = [];
          while (font.pos < end) {
            const stringLength = font.getByte();