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.__originalTransform = ctx.transform;
  ctx.__originalSetTransform = ctx.setTransform;
  ctx.__originalResetTransform = ctx.resetTransform;
  ctx.__originalClip = ctx.clip;
  ctx.__originalMoveTo = ctx.moveTo;
  ctx.__originalLineTo = ctx.lineTo;
  ctx.__originalBezierCurveTo = ctx.bezierCurveTo;
  ctx.__originalRect = ctx.rect;
  ctx.__originalClosePath = ctx.closePath;
  ctx.__originalBeginPath = ctx.beginPath;
  ctx._removeMirroring = () => {
    ctx.save = ctx.__originalSave;
    ctx.restore = ctx.__originalRestore;
    ctx.rotate = ctx.__originalRotate;
    ctx.scale = ctx.__originalScale;
    ctx.translate = ctx.__originalTranslate;
    ctx.transform = ctx.__originalTransform;
    ctx.setTransform = ctx.__originalSetTransform;
    ctx.resetTransform = ctx.__originalResetTransform;
    ctx.clip = ctx.__originalClip;
    ctx.moveTo = ctx.__originalMoveTo;
    ctx.lineTo = ctx.__originalLineTo;
    ctx.bezierCurveTo = ctx.__originalBezierCurveTo;
    ctx.rect = ctx.__originalRect;
    ctx.closePath = ctx.__originalClosePath;
    ctx.beginPath = ctx.__originalBeginPath;
    delete ctx._removeMirroring;
  };
  ctx.save = function ctxSave() {
    destCtx.save();