Created attachment 50667 [details] Web archive Steps to reproduce: 1. Visit the URL or open the attached web archive 2. Start scrolling down and to the right. I find that it is easier to reproduce with momentum scrolling Result: ASSERTION FAILED: !layer->m_zOrderListsDirty (WebCore/rendering/RenderLayerCompositor.cpp:689 void WebCore::RenderLayerCompositor::rebuildCompositingLayerTree(WebCore::RenderLayer*, const WebCore::CompositingState&, WTF::Vector<WebCore::GraphicsLayer*, 0ul>&))