Bug 316344 – [Site Isolation] Enable MultiProcessBackForwardCache in WebKitTestRunner when Site Isolation is enabled

• Status: RESOLVED
• Resolution: FIXED
• Component: Tools / Tests

[Basuke Suzuki]

Under Site Isolation, WebProcessPool::createWebPage auto-enables MultiProcessBackForwardCache (and UseUIProcessForBackForwardItemLoading) at page creation. WebKitTestRunner applies a full preference set per test (resetPreferencesToConsistentValues -> WKPreferencesResetTestRunnerOverrides), resetting unspecified preferences to their defaults and clobbering that runtime flip. As a result, BFCache-under-Site-Isolation layout tests run with MultiProcessBackForwardCacheEnabled=false, so BackForwardCache::addIfCacheable's guard (siteIsolationEnabled() && !multiProcessBackForwardCacheEnabled()) rejects caching and back/forward restore fails (e.g. fast/events/event-timing-back-forward-cache-duration.html under the mac-site-isolation suite).

Mirror the product coupling in WebKitTestRunner: when SiteIsolationEnabled is set, also enable MultiProcessBackForwardCacheEnabled. Add a same-origin Site-Isolation copy of the event-timing test as a regression guard.

UseUIProcessForBackForwardItemLoading coupling is deferred; enabling it surfaces a separate nested-frame form-state restoration failure.

[Basuke Suzuki]

<rdar://problem/178760818>

[Basuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/66491

[EWS]

Committed 314591@main (49a03a1fa5d5): <https://commits.webkit.org/314591@main>

Reviewed commits have been landed. Closing PR #66491 and removing active labels.