[New Multicolumn] Only the nearest ancestral flow thread may serve as a containing flow thread.

Source/core/layout/LayoutState.cpp

Index: Source/core/layout/LayoutState.cpp
diff --git a/Source/core/layout/LayoutState.cpp b/Source/core/layout/LayoutState.cpp
index 6f94933777207c0135cb8775a6a04cf7eb9a17a8..c14f2dfb4dae29220ad6cacb371d6a98f72a7175 100644
--- a/Source/core/layout/LayoutState.cpp
+++ b/Source/core/layout/LayoutState.cpp
@@ -52,7 +52,12 @@ LayoutState::LayoutState(LayoutBox& renderer, const LayoutSize& offset, LayoutUn
     , m_next(renderer.view()->layoutState())
     , m_renderer(renderer)
 {
-    m_flowThread = renderer.isLayoutFlowThread() ? toLayoutFlowThread(&renderer) : m_next->flowThread();
+    if (renderer.isLayoutFlowThread())
+        m_flowThread = toLayoutFlowThread(&renderer);
+    else if (!renderer.isOutOfFlowPositioned() && !renderer.isColumnSpanAll())
+        m_flowThread = m_next->flowThread();
+    else
+        m_flowThread = nullptr;
     renderer.view()->pushLayoutState(*this);
     bool fixed = renderer.isOutOfFlowPositioned() && renderer.style()->position() == FixedPosition;
     if (fixed) {