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 layerOffsetX = layerBox[0];
  const layerOffsetY = layerBox[1];
  const layerWidth = layerBox[2] - layerOffsetX;
  const layerHeight = layerBox[3] - layerOffsetY;
  if (layerWidth === 0 || layerHeight === 0) {
    return;
  }
  genericComposeSMask(
    smask.context,
    layerCtx,
    layerWidth,
    layerHeight,
    smask.subtype,
    smask.backdrop,
    smask.transferMap,
    layerOffsetX,
    layerOffsetY,
    smask.offsetX,
    smask.offsetY
  );
  ctx.save();
  ctx.globalAlpha = 1;
  ctx.globalCompositeOperation = "source-over";
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.drawImage(layerCtx.canvas, 0, 0);
  ctx.restore();
}
function getImageSmoothingEnabled(transform, interpolate) {