It has been intermittently failing on Mac & EFL: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20-%20webkit.org&tests=fast%2Fframes%2Fflattening%2Fiframe-flattening-fixed-height.html
Itβs somewhat red-herring that this test is relying on setTimeout(~, 0): 11 setTimeout(function() { 12 if (window.layoutTestController) 13 layoutTestController.notifyDone(); 14 }, 0); Why is that needed?