I hitted this assertion on master (r235459 without patches) a few seconds after I opened regular YouTube, with the expected slowness from debug mode. ASSERTION FAILED: !m_nicosia.imageBacking ../../Source/WebCore/platform/graphics/texmap/coordinated/CoordinatedGraphicsLayer.cpp(141) : virtual WebCore::CoordinatedGraphicsLayer::~CoordinatedGraphicsLayer() 1 0x7fb74845072a /webkit/WebKitBuild/Debug/lib/libjavascriptcoregtk-4.0.so.18(WTFCrash+0x9) [0x7fb74845072a] 2 0x7fb75335818d /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::CoordinatedGraphicsLayer::~CoordinatedGraphicsLayer()+0xa3) [0x7fb75335818d] 3 0x7fb75335827c /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::CoordinatedGraphicsLayer::~CoordinatedGraphicsLayer()+0x18) [0x7fb75335827c] 4 0x7fb753297cda /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::default_delete<WebCore::GraphicsLayer>::operator()(WebCore::GraphicsLayer*) const+0x2e) [0x7fb753297cda] 5 0x7fb75330fb3d /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::GraphicsLayer, std::default_delete<WebCore::GraphicsLayer> >::reset(WebCore::GraphicsLayer*)+0x55) [0x7fb75330fb3d] 6 0x7fb75330ee0d /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::GraphicsLayer, std::default_delete<WebCore::GraphicsLayer> >::operator=(decltype(nullptr))+0x21) [0x7fb75330ee0d] 7 0x7fb755182fca /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayerBacking::destroyGraphicsLayers()+0xa0) [0x7fb755182fca] 8 0x7fb755182365 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayerBacking::~RenderLayerBacking()+0xff) [0x7fb755182365] 9 0x7fb755182466 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayerBacking::~RenderLayerBacking()+0x18) [0x7fb755182466] 10 0x7fb7551aedd3 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::default_delete<WebCore::RenderLayerBacking>::operator()(WebCore::RenderLayerBacking*) const+0x23) [0x7fb7551aedd3] 11 0x7fb7551b0d05 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::RenderLayerBacking, std::default_delete<WebCore::RenderLayerBacking> >::reset(WebCore::RenderLayerBacking*)+0x55) [0x7fb7551b0d05] 12 0x7fb7551abb6f /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::RenderLayerBacking, std::default_delete<WebCore::RenderLayerBacking> >::operator=(decltype(nullptr))+0x21) [0x7fb7551abb6f] 13 0x7fb75517dbcd /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayer::clearBacking(bool)+0x87) [0x7fb75517dbcd] 14 0x7fb7551643de /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayer::~RenderLayer()+0x1ee) [0x7fb7551643de] 15 0x7fb75516458c /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayer::~RenderLayer()+0x18) [0x7fb75516458c] 16 0x7fb7551b395f /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::default_delete<WebCore::RenderLayer>::operator()(WebCore::RenderLayer*) const+0x23) [0x7fb7551b395f] 17 0x7fb7551b39b7 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::RenderLayer, std::default_delete<WebCore::RenderLayer> >::reset(WebCore::RenderLayer*)+0x55) [0x7fb7551b39b7] 18 0x7fb7551add5f /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::RenderLayer, std::default_delete<WebCore::RenderLayer> >::operator=(decltype(nullptr))+0x21) [0x7fb7551add5f] 19 0x7fb75519d524 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayerModelObject::destroyLayer()+0xbe) [0x7fb75519d524] 20 0x7fb75519d44b /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderLayerModelObject::willBeDestroyed()+0x87) [0x7fb75519d44b] 21 0x7fb7550a839f /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderBoxModelObject::willBeDestroyed()+0x89) [0x7fb7550a839f] 22 0x7fb75508fbb2 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderBox::willBeDestroyed()+0xba) [0x7fb75508fbb2] 23 0x7fb75504525c /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderBlockFlow::willBeDestroyed()+0x130) [0x7fb75504525c] 24 0x7fb7551f13db /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderObject::destroy()+0x105) [0x7fb7551f13db] 25 0x7fb7551ec306 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderObjectDeleter::operator()(WebCore::RenderObject*) const+0x1c) [0x7fb7551ec306] 26 0x7fb7550cfb35 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(std::unique_ptr<WebCore::RenderObject, WebCore::RenderObjectDeleter>::~unique_ptr()+0x49) [0x7fb7550cfb35] 27 0x7fb7553b5b65 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderTreeBuilder::destroy(WebCore::RenderObject&)+0x1c7) [0x7fb7553b5b65] 28 0x7fb7553b869c /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderTreeBuilder::destroyAndCleanUpAnonymousWrappers(WebCore::RenderObject&)+0xf4) [0x7fb7553b869c] 29 0x7fb7553cb2b3 /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(+0xb51a2b3) [0x7fb7553cb2b3] 30 0x7fb7553cb55f /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderTreeUpdater::tearDownRenderers(WebCore::Element&, WebCore::RenderTreeUpdater::TeardownType, WebCore::RenderTreeBuilder&)+0x239) [0x7fb7553cb55f] 31 0x7fb7553ca03c /webkit/WebKitBuild/Debug/lib/libwebkit2gtk-4.0.so.37(WebCore::RenderTreeUpdater::updateElementRenderer(WebCore::Element&, WebCore::Style::ElementUpdate const&)+0xe8) [0x7fb7553ca03c]