blink::IntersectionObserver includes a "delay" feature which will throttle change notifications. However, if an update is throttled, the observer will not then be notified at a later time, after the delay period has passed.

It seems that blink::IntersectionObservation will need to set a timer to call its Compute() method again after the delay period has passed, to ensure that an update eventually results in a notification to the observer.

As of this writing, it doesn't seem that any code is actually using the delay feature; so, P3.