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 ctx = this.ctx;
    const current = this.current;
    let x = current.x,
      y = current.y;
    let startX, startY;
    for (let i = 0, j = 0, ii = ops.length; i < ii; i++) {
      switch (ops[i] | 0) {
        case OPS.rectangle:
          x = args[j++];
          y = args[j++];
          const width = args[j++];
          const height = args[j++];
          const xw = x + width;
          const yh = y + height;
          ctx.moveTo(x, y);
          if (width === 0 || height === 0) {
            ctx.lineTo(xw, yh);
          } else {
            ctx.lineTo(xw, y);
            ctx.lineTo(xw, yh);
            ctx.lineTo(x, yh);
          }
          current.updatePathMinMax(ctx.mozCurrentTransform, x, y);
          current.updatePathMinMax(ctx.mozCurrentTransform, xw, yh);
          ctx.closePath();
          break;
        case OPS.moveTo:
          x = args[j++];
          y = args[j++];
          ctx.moveTo(x, y);