Semi-frequently seeing this on random layout tests: Thread 25 Crashed:: WebCore: Scrolling 0 com.apple.JavaScriptCore 0x0000000107afebda WTFCrash + 42 (Assertions.cpp:333) 1 com.apple.WebCore 0x000000010a4b37e4 WebCore::ScrollingCoordinatorMac::createScrollingTreeNode(WebCore::ScrollingNodeType, unsigned long long) + 100 (ScrollingCoordinatorMac.mm:147) 2 com.apple.WebCore 0x000000010995eadf WebCore::ThreadedScrollingTree::createNode(WebCore::ScrollingNodeType, unsigned long long) + 95 (ThreadedScrollingTree.cpp:125) 3 com.apple.WebCore 0x000000010a4c4c4e WebCore::ScrollingTree::updateTreeFromStateNode(WebCore::ScrollingStateNode const*) + 462 (ScrollingTree.cpp:134) 4 com.apple.WebCore 0x000000010a4c4983 WebCore::ScrollingTree::commitNewTreeState(WTF::PassOwnPtr<WebCore::ScrollingStateTree>) + 563 (ScrollingTree.cpp:107) 5 com.apple.WebCore 0x000000010995e7e0 WebCore::ThreadedScrollingTree::commitNewTreeState(WTF::PassOwnPtr<WebCore::ScrollingStateTree>) + 112 (ThreadedScrollingTree.cpp:100) 6 com.apple.WebCore 0x000000010a4b4706 WTF::FunctionWrapper<void (WebCore::ThreadedScrollingTree::*)(WTF::PassOwnPtr<WebCore::ScrollingStateTree>)>::operator()(WebCore::ThreadedScrollingTree*, WTF::PassOwnPtr<WebCore::ScrollingStateTree>) + 150 (Functional.h:246) 7 com.apple.WebCore 0x000000010a4b4635 WTF::BoundFunctionImpl<WTF::FunctionWrapper<void (WebCore::ThreadedScrollingTree::*)(WTF::PassOwnPtr<WebCore::ScrollingStateTree>)>, void (WebCore::ThreadedScrollingTree*, WTF::PassOwnPtr<WebCore::ScrollingStateTree>)>::operator()() + 101 (Functional.h:522) 8 com.apple.WebCore 0x00000001096c35f2 WTF::Function<void ()>::operator()() const + 114 (Functional.h:704) 9 com.apple.WebCore 0x000000010a4c2d23 WebCore::ScrollingThread::dispatchFunctionsFromScrollingThread() + 195 (ScrollingThread.cpp:116) http://build.webkit.org/results/Apple%20MountainLion%20Debug%20WK2%20(Tests)/r161937%20(15302)/platform/mac-wk2/tiled-drawing/fixed-background/fixed-body-background-crash-log.txt