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.strokeStyle = "#000000";
  ctx.fillStyle = "#000000";
  ctx.fillRule = "nonzero";
  ctx.globalAlpha = 1;
  ctx.lineWidth = 1;
  ctx.lineCap = "butt";
  ctx.lineJoin = "miter";
  ctx.miterLimit = 10;
  ctx.globalCompositeOperation = "source-over";
  ctx.font = "10px sans-serif";
  if (ctx.setLineDash !== undefined) {
    ctx.setLineDash([]);
    ctx.lineDashOffset = 0;
  }
}
function composeSMaskBackdrop(bytes, r0, g0, b0) {