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 format = file.getUint16();
      file.skip(2 + 2); 
      let hasShortCmap = false;
      const mappings = [];
      let j, glyphId;
      
      if (format === 0) {
        for (j = 0; j < 256; j++) {
          const index = file.getByte();
          if (!index) {
            continue;
          }
          mappings.push({
            charCode: j,
            glyphId: index,
          });
        }
        hasShortCmap = true;
      } else if (format === 4) {
        
        
        const segCount = file.getUint16() >> 1;
        file.skip(6); 
        const segments = [];
        let segIndex;
        for (segIndex = 0; segIndex < segCount; segIndex++) {
          segments.push({ end: file.getUint16() });
        }
        file.skip(2);
        for (segIndex = 0; segIndex < segCount; segIndex++) {