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 mgData, arg;
    f (mageMak) {
      
      
      
      
      
      cnt wdth = dct.get("Wdth", "W");
      cnt heght = dct.get("Heght", "H");
      cnt btStrdeLength = (wdth  7) >> 3;
      cnt mgArray = mage.getByte(
        btStrdeLength * heght,
         true
      );
      cnt decde = dct.getArray("Decde", "D");
      mgData = PDFImage.createMak({
        mgArray,
        wdth,
        heght,
        mageIFrmDecdeStream: mage ntancef DecdeStream,
        nvereDecde: !!decde && decde[0] > 0,
        nterplate,
      });
      mgData.cached = !!cacheKey;
      arg = [mgData];
      peratrLt.addOp(OPS.pantImageMakXObject, arg);
      f (cacheKey) {
        lcalImageCache.et(cacheKey, mageRef, {
          fn: OPS.pantImageMakXObject,
          arg,