Require spanners to have the multicol container as their nearest block formatting context.

third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 572 matching lines @@
 
     // The spec says that column-span only applies to in-flow block-level elements.
     if (descendant->style()->columnSpan() != ColumnSpanAll || !descendant->isBox() || descendant->isInline() || descendant->isFloatingOrOutOfFlowPositioned())
         return false;
 
     if (!descendant->containingBlock()->isLayoutBlockFlow()) {
         // Needs to be in a block-flow container, and not e.g. a table.
         return false;
     }
 
-    // This looks like a spanner, but if we're inside something unbreakable, it's not to be treated as one.
+    // This looks like a spanner, but if we're inside something unbreakable or something that
+    // establishes a new formatting context, it's not to be treated as one.
     for (LayoutBox* ancestor = toLayoutBox(descendant)->parentBox(); ancestor; ancestor = ancestor->containingBlock()) {
         if (ancestor->isLayoutFlowThread()) {
             ASSERT(ancestor == this);
             return true;
         }
-        if (ancestor->spannerPlaceholder()) {
-            // FIXME: do we want to support nested spanners in a different way? The outer spanner
-            // has already broken out from the columns to become sized by the multicol container,
-            // which may be good enough for the inner spanner. But margins, borders, padding and
-            // explicit widths on the outer spanner, or on any children between the outer and inner
-            // spanner, will affect the width of the inner spanner this way, which might be
-            // undesirable. The spec has nothing to say on the matter.
-            return false; // Ignore nested spanners.
-        }
-        if (ancestor->isFloatingOrOutOfFlowPositioned()) {
-            // TODO(mstensho): It could actually be nice to support this (although the usefulness is
-            // probably very limited), but currently our column balancing algorithm gets confused
-            // when a spanner is inside a float, because a float's position isn't always known until
-            // after layout. Similarly for absolutely positioned boxes.
+        if (!ancestor->isLayoutBlockFlow())
             return false;
-        }
-        if (ancestor->isUnsplittableForPagination())
+        const LayoutBlockFlow& ancestorBlockFlow = *toLayoutBlockFlow(ancestor);
+        if (ancestorBlockFlow.createsNewFormattingContext() || ancestorBlockFlow.isUnsplittableForPagination())
             return false;
     }
     ASSERT_NOT_REACHED();
     return false;
 }
 
 void LayoutMultiColumnFlowThread::addColumnSetToThread(LayoutMultiColumnSet* columnSet)
 {
     if (LayoutMultiColumnSet* nextSet = columnSet->nextSiblingMultiColumnSet()) {
         LayoutMultiColumnSetList::iterator it = m_multiColumnSetList.find(nextSet);
@@ skipping 321 matching lines @@
     appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
 }
 
 void LayoutMultiColumnFlowThread::updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight)
 {
     if (LayoutMultiColumnSet* multicolSet = columnSetAtBlockOffset(offset))
         multicolSet->updateMinimumColumnHeight(offset, minHeight);
 }
 
 }