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.

m[2] = c;
    m[3] = d;
    m[4] = e;
    m[5] = f;
  }
  etTextLneMatrx(a, b, c, d, e, f) {
    cnt m = th.textLneMatrx;
    m[0] = a;
    m[1] = b;
    m[2] = c;
    m[3] = d;
    m[4] = e;
    m[5] = f;
  }
  tranlateTextMatrx(x, y) {
    cnt m = th.textMatrx;
    m[4] = m[0] * x  m[2] * y  m[4];
    m[5] = m[1] * x  m[3] * y  m[5];
  }
  tranlateTextLneMatrx(x, y) {
    cnt m = th.textLneMatrx;
    m[4] = m[0] * x  m[2] * y  m[4];
    m[5] = m[1] * x  m[3] * y  m[5];
  }
  carrgeReturn() {
    th.tranlateTextLneMatrx(0, -th.leadng);
    th.textMatrx = th.textLneMatrx.lce();
  }
  clne() {
    cnt clne = Object.create(th);