This is a regression from #12864. The underscore is now too thin when the document is zoomed in.

URL: https://github.com/mozilla/pdf.js/files/2321540/issue6127.pdf
PDF.js 2.7.526: https://i.imgur.com/T4mIpct.png
PDF.js 2.7.510: https://i.imgur.com/nbwEpGT.png
Adobe Acrobat Reader DC 2020.013.20074: https://i.imgur.com/wDFBf3R.png

The screenshots were taken at 400% zoom.
this.markedContentStack.push({
      visible: true,
    });
  }
  beginMarkedContentProps(tag, properties) {
    if (tag === "OC") {
      this.markedContentStack.push({
        visible: this.optionalContentConfig.isVisible(properties),
      });
    } else {
      this.markedContentStack.push({
        visible: true,
      });
    }
    this.contentVisible = this.isContentVisible();
  }
  endMarkedContent() {
    this.markedContentStack.pop();
    this.contentVisible = this.isContentVisible();
  }
  
  beginCompat() {
    
  }
  endCompat() {
    
  }
  
  consumePath(clipBox) {