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.ctx.save();
    const old = this.current;
    this.stateStack.push(old);
    this.current = old.clone();
  }
  restore() {
    if (this.stateStack.length === 0 && this.current.activeSMask) {
      this.endSMaskMode();
    }
    if (this.stateStack.length !== 0) {
      this.current = this.stateStack.pop();
      this.ctx.restore();
      this.checkSMaskState();
      
      this.pendingClip = null;
      this._cachedGetSinglePixelWidth = null;
    }
  }
  transform(a, b, c, d, e, f) {
    this.ctx.transform(a, b, c, d, e, f);
    this._cachedGetSinglePixelWidth = null;
  }
  
  constructPath(ops, args) {