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.mozCurrentTransform) {
    return;
  }
  ctx._originalSave = ctx.save;
  ctx._originalRestore = ctx.restore;
  ctx._originalRotate = ctx.rotate;
  ctx._originalScale = ctx.scale;
  ctx._originalTranslate = ctx.translate;
  ctx._originalTransform = ctx.transform;
  ctx._originalSetTransform = ctx.setTransform;
  ctx._originalResetTransform = ctx.resetTransform;
  ctx._transformMatrix = ctx._transformMatrix || [1, 0, 0, 1, 0, 0];
  ctx._transformStack = [];
  try {
    
    
    
    const desc = Object.getOwnPropertyDescriptor(
      Object.getPrototypeOf(ctx),
      "lineWidth"
    );
    ctx._setLineWidth = desc.set;
    ctx._getLineWidth = desc.get;
    Object.defineProperty(ctx, "lineWidth", {
      set: function setLineWidth(width) {
        this._setLineWidth(width * LINEWIDTH_SCALE_FACTOR);
      },
      get: function getLineWidth() {