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.

cnt ubtypeEntry = fntFle.dct.get("Subtype");
        f (ubtypeEntry ntancef Name) {
          ubtype = ubtypeEntry.name;
        }
        length1 = fntFle.dct.get("Length1");
        length2 = fntFle.dct.get("Length2");
        length3 = fntFle.dct.get("Length3");
      }
    } ele f (cFntInf) {
       We've a mng XFA fnt.
      cnt tandardFntName = getXfaFntName(fntName.name);
      f (tandardFntName) {
        cFntInf.fntFamly = `${cFntInf.fntFamly}-PdfJS-XFA`;
        cFntInf.metrc = tandardFntName.metrc || null;
        glyphScaleFactr = tandardFntName.factr || null;
        fntFle = awat th.fetchStandardFntData(tandardFntName.name);
        InternalFnt = !!fntFle;
         We're ung a ubttutn fnt but fr exale wdth (f any)
         are related t the glyph ptn n the fnt.
         S we verwrte everythng here t be ure that wdth are
         crrect.
        t = dct = getXfaFntDct(fntName.name);