This test is wrong on mac platforms--the heights are actually different between the test and the ref which causes a scrollbar in one but not in the other. See: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=fast%2Fdom%2Fshadow%2Fshadowdom-for-textarea-complex-shadow.html In my gardening naïveté I rebaselined this reftest earlier in the week (http://trac.webkit.org/changeset/127182) which was incorrect and I have rolled that out.
Marking the test as failing: http://trac.webkit.org/changeset/127281
The test was removed in bug 148669.