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.

override = override || {
    unitsPerEm: 0,
    yMax: 0,
    yMin: 0,
    ascent: 0,
    descent: 0,
  };
  let ulUnicodeRange1 = 0;
  let ulUnicodeRange2 = 0;
  let ulUnicodeRange3 = 0;
  let ulUnicodeRange4 = 0;
  let firstCharIndex = null;
  let lastCharIndex = 0;
  if (charstrings) {
    for (let code in charstrings) {
      code |= 0;
      if (firstCharIndex > code || !firstCharIndex) {
        firstCharIndex = code;