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.current.strokeColor = this.getColorN_Pattern(arguments);
  }
  setFillColorN() {
    this.current.fillColor = this.getColorN_Pattern(arguments);
    this.current.patternFill = true;
  }
  setStrokeRGBColor(r, g, b) {
    const color = Util.makeHexColor(r, g, b);
    this.ctx.strokeStyle = color;
    this.current.strokeColor = color;
  }
  setFillRGBColor(r, g, b) {
    const color = Util.makeHexColor(r, g, b);
    this.ctx.fillStyle = color;
    this.current.fillColor = color;
    this.current.patternFill = false;
  }
  _getPattern(objId, matrix = null) {