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.

const tables = Object.create(null);
      tables["OS/2"] = null;
      tables.cmap = null;
      tables.head = null;
      tables.hhea = null;
      tables.hmtx = null;
      tables.maxp = null;
      tables.name = null;
      tables.post = null;
      for (let i = 0; i < numTables; i++) {
        const table = readTableEntry(file);
        if (!VALID_TABLES.includes(table.tag)) {
          continue; 
        }
        if (table.length === 0) {
          continue; 
        }
        tables[table.tag] = table;
      }
      return tables;
    }
    function readTableEntry(file) {