316345 2026-06-04 15:08:29 -0700 [Site Isolation] Form state in nested frames is lost when restoring from back/forward cache with UseUIProcessForBackForwardItemLoading 2026-06-05 10:35:23 -0700 1 1 1 Unclassified WebKit WebKit Misc. WebKit Nightly Build Unspecified Unspecified RESOLVED FIXED InRadar P2 Normal --- 1 basuke basuke webkit-bug-importer oldest_to_newest 2217751 0 basuke 2026-06-04 15:08:29 -0700 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. 2217755 1 basuke 2026-06-04 15:11:47 -0700 <rdar://problem/178762633> 2217756 2 basuke 2026-06-04 15:12:06 -0700 Pull request: https://github.com/WebKit/WebKit/pull/66494 2218053 3 ews-feeder 2026-06-05 10:35:21 -0700 Committed 314648@main (1cff9b97d680): <https://commits.webkit.org/314648@main> Reviewed commits have been landed. Closing PR #66494 and removing active labels.