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.

const firstEntryLength = itemDecode(locaData, itemSize);
        if (newGlyfData.length > firstEntryLength + writeOffset) {
          glyf.data = newGlyfData.subarray(0, firstEntryLength + writeOffset);
        } else {
          glyf.data = new Uint8Array(firstEntryLength + writeOffset);
          glyf.data.set(newGlyfData.subarray(0, writeOffset));
        }
        glyf.data.set(newGlyfData.subarray(0, firstEntryLength), writeOffset);
        itemEncode(
          loca.data,
          locaData.length - itemSize,
          writeOffset + firstEntryLength
        );
      } else {
        glyf.data = newGlyfData.subarray(0, writeOffset);
      }
      return {
        missingGlyphs,
        maxSizeOfInstructions,
      };
    }
    function readPostScriptTable(post, propertiesObj, maxpNumGlyphs) {
      const start = (font.start ? font.start : 0) + post.offset;
      font.pos = start;
      const length = post.length,
        end = start + length;
      const version = font.getInt32();