Detecting whether a navigation was BFCache-eligible

[fergald]

In order to produce accurate BFCache hit-rate stats a page must be able to tell whether a navigation was even eligible for BFCache in the first place. Currently there is not enough information available from APIs to tell this. Pages which are the result of a session-restore or a tab-clone of a history navigation cannot be distinguished from genuine history navigation.

Steps to demonstrate the issue

1. go to a page (A) that cannot be BFCached
2. go to any other page (B)
3. go back to A
4. In JS, execute performance.getEntriesByType('navigation')[0].type and you will see back_forward.
5. clone the tab or quit the browser and restart
6. In JS, execute performance.getEntriesByType('navigation')[0].type and you will still see back_forward.

Session cookies could be used to detect a session restore. I can't think of anything that could be used to detect a tab-clone.

Should we have something else to indicate that the current navigation is from a session-restore or tab clone?

The proposed not-restored reason API could expose this e.g. by presenting an "ineligible" reason when the last navigation was ineligible for BFCache. Perhaps it would be simpler and more useful to present directly whether the current page came from restore/clone. This would provide more information, in the case where the previous navigation was not back_forward since those navigations are also ineligible regardless of session-restore/clone.

@domenic @yoavweiss

[noamr]

Does wasDiscurded help you here? This seems somewhat related to https://github.com/w3c/navigation-timing/issues/178

[fergald]

@noamr sorry, I left this unsent for a long time.

I don't think so. I could imagine that it might tell in the case of a session restore (but in my testing with Chrome it is not true on session restore) but for a tab clone, I don't think there would be any expectation that it might be true.