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.
if (font.disableFontFace || patternFill) {
      ctx.save();
      ctx.translate(x, y);
      ctx.beginPath();
      addToPath(ctx, fontSize);
      if (patternTransform) {
        ctx.setTransform.apply(ctx, patternTransform);
      }
      if (
        fillStrokeMode === TextRenderingMode.FILL ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {
        ctx.fill();
      }
      if (
        fillStrokeMode === TextRenderingMode.STROKE ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {
        if (resetLineWidthToOne) {
          ctx.resetTransform();
          ctx.lineWidth = Math.round(this._combinedScaleFactor);
        }
        ctx.stroke();
      }
      ctx.restore();
    } else {
      if (
        fillStrokeMode === TextRenderingMode.FILL ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {