Created attachment 38900 [details]
Sometimes this assertion fires:
ASSERTION FAILED: !repaintContainer || repaintContainer == this
(/Volumes/InternalData/Development/webkit/OpenSource/WebCore/rendering/RenderView.cpp:269 virtual void WebCore::RenderView::computeRectForRepaint(WebCore::RenderBoxModelObject*, WebCore::IntRect&, bool))
One cause is content that has an element with opacity between two positioned elements in the hierarchy, because container() jumps over that opacity layer.
*** This bug has been marked as a duplicate of bug 29755 ***