https://w3c.github.io/compute-pressure/#handling-change-of-fully-active lists what to do when a Document either (1) stops being fully active or (2) starts being fully active again.

The steps for (1) basically say that data delivery must stop, which is pretty vague: should any hardware resources or platform handles be released, for example?

The steps for (2) are even more unclear: if telemetry collection is stopped at the platform level in (1), it means that reactivating collection would mean going through most of https://w3c.github.io/compute-pressure/#the-observe-method without calling the method itself. This includes the fact that starting telemetry data collection may fail for multiple reasons.

Maybe (2) shouldn't exist at all; however, if we do this the API should have a way to signal that it's no longer going to provide updates so that when the document is restored by the bfcache existing PressureObserver instances just remain existing without providing any data.

It's also curious that there doesn't seem to be a way to distinguish between a document not being fully active because e.g. it's in an iframe that got detached (and thus will never become fully active again, so we could just drop all resources without caring) and it becoming fully active because it's entering the bfcache (in which case we could care about gathering data automatically again).