Refactor avoidsFloats() to reflect CSS2.1 spec more clearly

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 4060 matching lines @@
     // Floating objects don't shrink.  Objects that don't avoid floats don't shrink.  Marquees don't shrink.
     if ((isInline() && !isMarquee()) || !avoidsFloats() || isFloating())
         return false;
 
     // Only auto width objects can possibly shrink to avoid floats.
     return style()->width().isAuto();
 }
 
 bool RenderBox::avoidsFloats() const
 {
-    return isReplaced() || hasOverflowClip() || isHR() || isLegend() || isWritingModeRoot() || isFlexItemIncludingDeprecated();
+    // CSS2.1: "The border box of a table, a block-level replaced element, or an element in the normal flow that establishes a new block formatting
+    // context .. must not overlap the margin box of any floats in the same block formatting context."
+    // FIXME: The inclusion of horizontal rule and legend elements here isn't covered by any spec.
+    return isReplaced() || isHR() || isLegend() || isTable() || (!isFloatingOrOutOfFlowPositioned() && createsBlockFormattingContext());
+}
+
+bool RenderBox::createsBlockFormattingContext() const
+{
+    return isInlineBlockOrInlineTable() || isFloatingOrOutOfFlowPositioned() || hasOverflowClip() || isFlexItemIncludingDeprecated()
+        || style()->specifiesColumns() || isRenderFlowThread() || isTableCell() || isTableCaption() || isFieldset() || isWritingModeRoot() || isDocumentElement() || style()->columnSpan();
 }
 
 void RenderBox::markForPaginationRelayoutIfNeeded(SubtreeLayoutScope& layoutScope)
 {
     ASSERT(!needsLayout());
     // If fragmentation height has changed, we need to lay out. No need to enter the renderer if it
     // is childless, though.
     if (view()->layoutState()->pageLogicalHeightChanged() && slowFirstChild())
         layoutScope.setChildNeedsLayout(this);
 }
@@ skipping 543 matching lines @@
         return 0;
 
     if (!layoutState && !flowThreadContainingBlock())
         return 0;
 
     RenderBlock* containerBlock = containingBlock();
     return containerBlock->offsetFromLogicalTopOfFirstPage() + logicalTop();
 }
 
 } // namespace WebCore