[New Multicolumn] Handle overflow inside spanners correctly.

Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp

Index: Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
diff --git a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
index b0555a5b2af6c9766a4a282382d2e39e16ee7c6e..169146bfc2519ed72f58ca137b3a70247902da0b 100644
--- a/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
+++ b/Source/core/rendering/RenderMultiColumnSpannerPlaceholder.cpp
@@ -55,6 +55,12 @@ void RenderMultiColumnSpannerPlaceholder::layout()
     // 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();
 }