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.
if (ctx._removeMirroring) {
    throw new Error("Context is already forwarding operations.");
  }
  ctx.__originalSave = ctx.save;
  ctx.__originalRestore = ctx.restore;
  ctx.__originalRotate = ctx.rotate;
  ctx.__originalScale = ctx.scale;
  ctx.__originalTranslate = ctx.translate;