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.
ctx.fillText(character, x, y);
      }
      if (
        fillStrokeMode === TextRenderingMode.STROKE ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {
        if (resetLineWidthToOne) {
          ctx.save();
          ctx.moveTo(x, y);
          ctx.resetTransform();
          ctx.lineWidth = Math.round(this._combinedScaleFactor);
          ctx.strokeText(character, 0, 0);
          ctx.restore();
        } else {
          ctx.strokeText(character, x, y);
        }
      }
    }
    if (isAddToPathSet) {
      const paths = this.pendingTextPaths || (this.pendingTextPaths = []);
      paths.push({
        transform: ctx.mozCurrentTransform,
        x,
        y,
        fontSize,
        addToPath,
      });
    }
  }
  get isFontSubpixelAAEnabled() {