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.
0,
      scaled.img.width,
      scaled.img.height,
      0,
      0,
      width,
      height
    );
    fillCtx.globalCompositeOperation = "source-in";
    const inverse = Util.transform(fillCtx.mozCurrentTransformInverse, [
      1,
      0,
      0,
      1,
      -offsetX,
      -offsetY,
    ]);
    fillCtx.fillStyle = isPatternFill
      ? fillColor.getPattern(ctx, this, inverse, PathType.FILL)
      : fillColor;
    fillCtx.fillRect(0, 0, width, height);
    
    return {
      canvas: fillCanvas.canvas,
      offsetX: Math.round(offsetX),
      offsetY: Math.round(offsetY),
    };
  }
  
  setLineWidth(width) {