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.lineWidth = width;
    this.ctx.lineWidth = width;
  }
  setLineCap(style) {
    this.ctx.lineCap = LINE_CAP_STYLES[style];
  }
  setLineJoin(style) {
    this.ctx.lineJoin = LINE_JOIN_STYLES[style];
  }
  setMiterLimit(limit) {
    this.ctx.miterLimit = limit;
  }
  setDash(dashArray, dashPhase) {
    const ctx = this.ctx;
    if (ctx.setLineDash !== undefined) {
      ctx.setLineDash(dashArray);
      ctx.lineDashOffset = dashPhase;
    }
  }
  setRenderingIntent(intent) {