[New Multicolumn] balancer confused by content with top margins.

Source/core/rendering/RenderBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 210 matching lines @@
             setLogicalHeight(oldLogicalHeight);
         }
         if (colInfo->columnHeight() != pageLogicalHeight && everHadLayout()) {
             colInfo->setColumnHeight(pageLogicalHeight);
             pageLogicalHeightChanged = true;
         }
 
         if (!hasSpecifiedPageLogicalHeight && !pageLogicalHeight)
             colInfo->clearForcedBreaks();
     } else if (isRenderFlowThread()) {
-        pageLogicalHeight = 1; // This is just a hack to always make sure we have a page logical height.
-        pageLogicalHeightChanged = toRenderFlowThread(this)->pageLogicalSizeChanged();
+        RenderFlowThread* flowThread = toRenderFlowThread(this);
+
+        // FIXME: This is a hack to always make sure we have a page logical height, if said height
+        // is known. The page logical height thing in LayoutState is meaningless for flow
+        // thread-based pagination (page height isn't necessarily uniform throughout the flow
+        // thread), but as long as it is used universally as a means to determine whether page
+        // height is known or not, we need this. Page height is unknown when column balancing is
+        // enabled and flow thread height is still unknown (i.e. during the first layout pass). When
+        // it's unknown, we need to prevent the pagination code from assuming page breaks everywhere
+        // and thereby eating every top margin. It should be trivial to clean up and get rid of this
+        // hack once the old multicol implementation is gone.
+        pageLogicalHeight = flowThread->isPageLogicalHeightKnown() ? LayoutUnit(1) : LayoutUnit(0);
+
+        pageLogicalHeightChanged = flowThread->pageLogicalSizeChanged();
     }
 }
 
 bool RenderBlockFlow::shouldRelayoutForPagination(LayoutUnit& pageLogicalHeight, LayoutUnit layoutOverflowLogicalBottom) const
 {
     // FIXME: We don't balance properly at all in the presence of forced page breaks. We need to understand what
     // the distance between forced page breaks is so that we can avoid making the minimum column height too tall.
     ColumnInfo* colInfo = columnInfo();
     LayoutUnit columnHeight = pageLogicalHeight;
     const int minColumnCount = colInfo->forcedBreaks() + 1;
@@ skipping 2570 matching lines @@
 RenderBlockFlow::RenderBlockFlowRareData& RenderBlockFlow::ensureRareData()
 {
     if (m_rareData)
         return *m_rareData;
 
     m_rareData = adoptPtr(new RenderBlockFlowRareData(this));
     return *m_rareData;
 }
 
 } // namespace WebCore