Bug 316345 – [Site Isolation] Form state in nested frames is lost when restoring from back/forward cache with UseUIProcessForBackForwardItemLoading

• Status: RESOLVED
• Resolution: FIXED
• Component: WebKit Misc.

[Basuke Suzuki]

When UseUIProcessForBackForwardItemLoading is enabled under Site Isolation (as it is in production via WebProcessPool::createWebPage), restoring a page that contains nested frames from the back/forward cache leaves the nested frames inaccessible: their contentDocument is null, so form state stored in nested frames is lost.

Reproduced by fast/loader/site-isolation/form-state-restore-with-frames.html once UseUIProcessForBackForwardItemLoading is coupled to Site Isolation in WebKitTestRunner. The test deterministically fails (5/5) with:
  FAIL frame$($("frame1"), "input2").value should be value2. Threw exception TypeError: null is not an object
  FAIL frame$(frame$($("frame1"), "frame2"), "input3").value should be value3. Threw exception TypeError: null is not an object (evaluating 'frame.contentDocument')

This was previously masked because the layout-test harness did not actually enable UseUIProcessForBackForwardItemLoading under Site Isolation: WebKitTestRunner's per-test preference reset clobbered the product's auto-enable (see bug 316344). Coupling the flag in the harness surfaces the underlying restoration defect.

EWS on the draft may reveal additional tests affected by the same root cause.

[Basuke Suzuki]

<rdar://problem/178762633>

[Basuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/66494

[EWS]

Committed 314648@main (1cff9b97d680): <https://commits.webkit.org/314648@main>

Reviewed commits have been landed. Closing PR #66494 and removing active labels.