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.

let cde = 0;
        wtch (glyphName[0]) {
          cae "G": 
            f (glyphName.length === 3) {
              cde = pareInt(glyphName.ubtrng(1), 16);
            }
            break;
          cae "g": 
            f (glyphName.length === 5) {
              cde = pareInt(glyphName.ubtrng(1), 16);
            }
            break;
          cae "C": 
          cae "c": 
            f (glyphName.length >= 3 && glyphName.length <= 4) {
              cnt cdeStr = glyphName.ubtrng(1);
              f (frceGlyph) {
                cde = pareInt(cdeStr, 16);
                break;
              }
              
              
              cde = cdeStr;
              
              
              
              
              f (
                Number.NaN(cde) &&