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

Source/core/rendering/LayoutState.cpp

 /*
  * Copyright (C) 2007 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 101 matching lines @@
     }
 
     if (!RuntimeEnabledFeatures::repaintAfterLayoutEnabled()) {
         m_layoutDelta = m_next->m_layoutDelta;
 #if !ASSERT_DISABLED
         m_layoutDeltaXSaturated = m_next->m_layoutDeltaXSaturated;
         m_layoutDeltaYSaturated = m_next->m_layoutDeltaYSaturated;
 #endif
     }
 
-    m_isPaginated = m_pageLogicalHeight || m_columnInfo || renderer.isRenderFlowThread();
+    m_isPaginated = m_pageLogicalHeight || m_columnInfo || renderer.flowThreadContainingBlock();

[abucur, 2014/03/25 10:02:36]

I think there's a chance you are breaking unsplittable content here such as
images / scrollable containers. m_isPaginated will be true for all the renderers
inside a flow thread because the inherited page height doesn't control this flag
any more. A couple of lines above, the page logical height is set to 0 for
unsplittable content but m_isPaginated will be true because
flowThreadContainingBlock() will be non-null.

Maybe you can group this isPaginated/isNotPaginated logic into a helper function
that should handle all the situations.

[mstensho (USE GERRIT), 2014/03/26 09:56:25]

Done.

 
     // FIXME: <http://bugs.webkit.org/show_bug.cgi?id=13443> Apply control clip if present.
 }
 
 LayoutState::LayoutState(RenderObject& root)
     : m_clipped(false)
     , m_isPaginated(false)
     , m_pageLogicalHeightChanged(false)
 #if !ASSERT_DISABLED
     , m_layoutDeltaXSaturated(false)
@@ skipping 44 matching lines @@
 }
 
 void LayoutState::addForcedColumnBreak(const RenderBox& child, const LayoutUnit& childLogicalOffset)
 {
     if (!m_columnInfo || m_columnInfo->columnHeight())
         return;
     m_columnInfo->addForcedBreak(pageLogicalOffset(child, childLogicalOffset));
 }
 
 } // namespace WebCore