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.
const width = canvas.width;
      const height = canvas.height;
      const bl = Util.applyTransform([0, 0], inv);
      const br = Util.applyTransform([0, height], inv);
      const ul = Util.applyTransform([width, 0], inv);
      const ur = Util.applyTransform([width, height], inv);
      const x0 = Math.min(bl[0], br[0], ul[0], ur[0]);
      const y0 = Math.min(bl[1], br[1], ul[1], ur[1]);
      const x1 = Math.max(bl[0], br[0], ul[0], ur[0]);
      const y1 = Math.max(bl[1], br[1], ul[1], ur[1]);
      this.ctx.fillRect(x0, y0, x1 - x0, y1 - y0);
    } else {
      
      
      
      
      
      this.ctx.fillRect(-1e10, -1e10, 2e10, 2e10);
    }
    this.compose(this.current.getClippedPathBoundingBox());
    this.restore();
  }
  
  beginInlineImage() {
    unreachable("Should not call beginInlineImage");
  }
  beginImageData() {
    unreachable("Should not call beginImageData");
  }
  paintFormXObjectBegin(matrix, bbox) {