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.
this.__originalSave();
  };
  ctx.restore = function ctxRestore() {
    destCtx.restore();
    this.__originalRestore();
  };
  ctx.translate = function ctxTranslate(x, y) {
    destCtx.translate(x, y);
    this.__originalTranslate(x, y);
  };
  ctx.scale = function ctxScale(x, y) {
    destCtx.scale(x, y);
    this.__originalScale(x, y);
  };
  ctx.transform = function ctxTransform(a, b, c, d, e, f) {
    destCtx.transform(a, b, c, d, e, f);
    this.__originalTransform(a, b, c, d, e, f);
  };
  ctx.setTransform = function ctxSetTransform(a, b, c, d, e, f) {
    destCtx.setTransform(a, b, c, d, e, f);
    this.__originalSetTransform(a, b, c, d, e, f);
  };
  ctx.resetTransform = function ctxResetTransform() {
    destCtx.resetTransform();
    this.__originalResetTransform();
  };
  ctx.rotate = function ctxRotate(angle) {
    destCtx.rotate(angle);
    this.__originalRotate(angle);
  };