Back-forward cache

The browser freezing a whole page — DOM, heap, scroll position — when you navigate away, and restoring it on back or forward. It makes history navigation effectively free, and it is disabled by a list of things: an `unload` handler, `no-store` on the document, an open connection the browser will not freeze. The `pageshow` event with `persisted` set is how you detect a restore and refresh anything that has gone stale.