[New Multicolumn] Handle overflow inside spanners correctly.

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/rendering/RenderMultiColumnSpannerPlaceholder.h"
 
 namespace blink {
 
 RenderMultiColumnSpannerPlaceholder* RenderMultiColumnSpannerPlaceholder::createAnonymous(RenderStyle* parentStyle, RenderBox* rendererInFlowThread)
@@ skipping 37 matching lines @@
 {
     ASSERT(needsLayout());
 
     // Lay out the actual column-span:all element.
     m_rendererInFlowThread->layoutIfNeeded();
 
     // The spanner has now been laid out, so its height is known. Time to update the placeholder's
     // height as well, so that we take up the correct amount of space in the multicol container.
     updateLogicalHeight();
 
+    // Take the overflow from the spanner, so that it gets
+    // propagated to the multicol container and beyond.
+    m_overflow.clear();
+    addVisualOverflow(m_rendererInFlowThread->visualOverflowRect());
+    addLayoutOverflow(m_rendererInFlowThread->layoutOverflowRect());
+
     clearNeedsLayout();
 }
 
 void RenderMultiColumnSpannerPlaceholder::computeLogicalHeight(LayoutUnit, LayoutUnit logicalTop, LogicalExtentComputedValues& computedValues) const
 {
     computedValues.m_extent = m_rendererInFlowThread->logicalHeight();
     computedValues.m_position = logicalTop;
 }
 
 void RenderMultiColumnSpannerPlaceholder::invalidateTreeIfNeeded(const PaintInvalidationState& paintInvalidationState)
@@ skipping 13 matching lines @@
 {
     return !m_rendererInFlowThread->hasSelfPaintingLayer() && m_rendererInFlowThread->nodeAtPoint(request, result, locationInContainer, accumulatedOffset, action);
 }
 
 const char* RenderMultiColumnSpannerPlaceholder::renderName() const
 {
     return "RenderMultiColumnSpannerPlaceholder";
 }
 
 }