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.

warn("cmap table has unsupported format: " + format);
        return {
          platformId: -1,
          encodingId: -1,
          mappings: [],
          hasShortCmap: false,
        };
      }
      
      mappings.sort(function (a, b) {
        return a.charCode - b.charCode;
      });
      for (let i = 1; i < mappings.length; i++) {
        if (mappings[i - 1].charCode === mappings[i].charCode) {
          mappings.splice(i, 1);
          i--;
        }
      }
      return {
        platformId: potentialTable.platformId,
        encodingId: potentialTable.encodingId,
        mappings,
        hasShortCmap,
      };
    }
    function sanitizeMetrics(
      file,
      header,
      metrics,
      headTable,