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 transform = this.ctx.mozCurrentTransform;
    return [
      transform[0] * x + transform[2] * y + transform[4],
      transform[1] * x + transform[3] * y + transform[5],
    ];
  }
  isContentVisible() {
    for (let i = this.markedContentStack.length - 1; i >= 0; i--) {
      if (!this.markedContentStack[i].visible) {
        return false;
      }
    }
    return true;
  }
}
for (const op in OPS) {
  if (CanvasGraphics.prototype[op] !== undefined) {
    CanvasGraphics.prototype[OPS[op]] = CanvasGraphics.prototype[op];
  }
}
export { CanvasGraphics };