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.

t["Times-Bold"] = "Times-Bold";
  t["Helvetica-Bold"] = "Helvetica-Bold";
  t["Courier-Bold"] = "Courier-Bold";
  t.ZapfDingbats = "ZapfDingbats";
  t["Times-Italic"] = "Times-Italic";
  t["Helvetica-Oblique"] = "Helvetica-Oblique";
  t["Courier-Oblique"] = "Courier-Oblique";
  t["Times-BoldItalic"] = "Times-BoldItalic";
  t["Helvetica-BoldOblique"] = "Helvetica-BoldOblique";
  t["Courier-BoldOblique"] = "Courier-BoldOblique";
  
  t.ArialNarrow = "Helvetica";
  t["ArialNarrow-Bold"] = "Helvetica-Bold";
  t["ArialNarrow-BoldItalic"] = "Helvetica-BoldOblique";
  t["ArialNarrow-Italic"] = "Helvetica-Oblique";
  t.ArialBlack = "Helvetica";
  t["ArialBlack-Bold"] = "Helvetica-Bold";
  t["ArialBlack-BoldItalic"] = "Helvetica-BoldOblique";
  t["ArialBlack-Italic"] = "Helvetica-Oblique";
  t["Arial-Black"] = "Helvetica";
  t["Arial-Black-Bold"] = "Helvetica-Bold";
  t["Arial-Black-BoldItalic"] = "Helvetica-BoldOblique";
  t["Arial-Black-Italic"] = "Helvetica-Oblique";
  t.Arial = "Helvetica";
  t["Arial-Bold"] = "Helvetica-Bold";
  t["Arial-BoldItalic"] = "Helvetica-BoldOblique";
  t["Arial-Italic"] = "Helvetica-Oblique";
  t["Arial-BoldItalicMT"] = "Helvetica-BoldOblique";
  t["Arial-BoldMT"] = "Helvetica-Bold";
  t["Arial-ItalicMT"] = "Helvetica-Oblique";