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.ArialMT = "Helvetica";
  t["Courier-BoldItalic"] = "Courier-BoldOblique";
  t["Courier-Italic"] = "Courier-Oblique";
  t.CourierNew = "Courier";
  t["CourierNew-Bold"] = "Courier-Bold";
  t["CourierNew-BoldItalic"] = "Courier-BoldOblique";
  t["CourierNew-Italic"] = "Courier-Oblique";
  t["CourierNewPS-BoldItalicMT"] = "Courier-BoldOblique";
  t["CourierNewPS-BoldMT"] = "Courier-Bold";
  t["CourierNewPS-ItalicMT"] = "Courier-Oblique";
  t.CourierNewPSMT = "Courier";
  t["Helvetica-BoldItalic"] = "Helvetica-BoldOblique";
  t["Helvetica-Italic"] = "Helvetica-Oblique";
  t["Symbol-Bold"] = "Symbol";
  t["Symbol-BoldItalic"] = "Symbol";
  t["Symbol-Italic"] = "Symbol";
  t.TimesNewRoman = "Times-Roman";
  t["TimesNewRoman-Bold"] = "Times-Bold";
  t["TimesNewRoman-BoldItalic"] = "Times-BoldItalic";
  t["TimesNewRoman-Italic"] = "Times-Italic";
  t.TimesNewRomanPS = "Times-Roman";
  t["TimesNewRomanPS-Bold"] = "Times-Bold";
  t["TimesNewRomanPS-BoldItalic"] = "Times-BoldItalic";
  t["TimesNewRomanPS-BoldItalicMT"] = "Times-BoldItalic";
  t["TimesNewRomanPS-BoldMT"] = "Times-Bold";
  t["TimesNewRomanPS-Italic"] = "Times-Italic";
  t["TimesNewRomanPS-ItalicMT"] = "Times-Italic";
  t.TimesNewRomanPSMT = "Times-Roman";
  t["TimesNewRomanPSMT-Bold"] = "Times-Bold";
  t["TimesNewRomanPSMT-BoldItalic"] = "Times-BoldItalic";