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 (!this.suspendedCtx) {
      throw new Error("endSMaskMode called while not in smask mode");
    }
    
    
    this.ctx._removeMirroring();
    copyCtxState(this.ctx, this.suspendedCtx);
    this.ctx = this.suspendedCtx;
    this.current.activeSMask = null;
    this.suspendedCtx = null;
  }
  compose(dirtyBox) {