Test if the cookie is locked during synchronous execution.

cookie-blocking-test.js

// Reading tab

var lastCookie;
setInterval(function (){
  if (lastCookie !== document.cookie) console.log("interval change");
  lastCookie = document.cookie;
  for (var i = 0; i < 10000; i++) {
    if (lastCookie !== document.cookie) console.log("change");
    lastCookie = document.cookie;
  }
}, 100);

// Writing tab

var i = 0;
setInterval(function (){
  i++;
  document.cookie = "x=" + i;
}, 100);