Currently, it seems like documents that are kept alive in the back-forward cache after we navigated away from it are treated as not fully active, and thus the document will be ignored in things that might update its contents/send messages to it, etc.

However, the fully active concept seems to depend on the fact that browsing context (and browsing context group) will never change across navigations. This is not true (at least in Chrome), where we will change browsing contexts (see: #5350).

This will also affect:

• WindowProxy, which will not change the Window object, since the browsing context is not navigated.
• When a method set by setTimeout, setInterval, etc. will be fired, since we're tracking the amount of time a document is fully active.

I wonder if it is enough to fix this by having a "fully active" concept on browsing contexts as well. So a fully active Document is only active if the browsing context it is in is fully active as well + the existing checks. Of course, this is blocked until after we actually update the spec to allow browsing context & browsing context group changes (which is tracked in #5350 I guess?).