[Webkit-unassigned] [Bug 250781] BFCache sporadically skips entries

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=250781

--- Comment #9 from David Dworken <ddworken at google.com> ---
After further investigation, we have come up with a minimal reproduction test case that demonstrates COOP changing the behavior of the back/forward button in the latest version of Safari. While this test case may seem convoluted, it is a direct corollary to a Google product where enabling COOP for Safari caused a breakage. The scenario is:

1. User is on some base page, that opens a popup to Page1
2. Page1 has a HTML form that auto-submits via JS to trigger a POST request to Page2
3. Page2 does a meta redirect to some final page
4. User hits the back button on the final page 

In this scenario, Safari will normally detect that Page1 and Page2 are redirects with no user interaction, and it will skip them when hitting the back button. Thus, hitting the back button will normally just close the popup. 

If COOP is enforced, Safari's heuristics here seem to fail which causes the user to get navigated back to Page1. Page1 then redirects the user to Page2 and then the final page, ultimately resulting in the user ending up on the same page as they started on, even when they hit the back button. 

You can observe this behavior by comparing the behavior of these two links in the latest version of Safari:

* https://pattern-dog-larkspur.glitch.me/safari-coop-bf-heuristic?coop=unsafe-none
* https://pattern-dog-larkspur.glitch.me/safari-coop-bf-heuristic?coop=same-origin-allow-popups

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