ex.

var root = document.createElement("div");
var observer = new IntersectionObserver(() => {}, {root:root});
root = null; // allow root to be collected since IO doesn't keep it alive.

while (true) {
  doSomething();
  try {
    observer.observe(null);
  } {
    // GC happened and not isRootValid() is false.
    break;
  }
}

You just need to make doSomething() do some work to potentially trigger a GC. This can also be used async, any time a method throws for an invalid root it leaks knowledge that a GC happened to the page.