[New Multicolumn] Make positionForPoint() work.

Source/core/layout/LayoutBlock.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
  * License as published by the Free Software Foundation; either
@@ skipping 2500 matching lines @@
     }
 
     // Can't reach this. We have a root line box, but it has no kids.
     // FIXME: This should ASSERT_NOT_REACHED(), but clicking on placeholder text
     // seems to hit this code path.
     return createPositionWithAffinity(0, DOWNSTREAM);
 }
 
 static inline bool isChildHitTestCandidate(LayoutBox* box)
 {
-    return box->size().height() && box->style()->visibility() == VISIBLE && !box->isFloatingOrOutOfFlowPositioned();
+    return box->size().height() && box->style()->visibility() == VISIBLE && !box->isFloatingOrOutOfFlowPositioned() && !box->isLayoutFlowThread();
 }
 
 PositionWithAffinity LayoutBlock::positionForPoint(const LayoutPoint& point)
 {
     if (isTable())
         return LayoutBox::positionForPoint(point);
 
     if (isReplaced()) {
         // FIXME: This seems wrong when the object's writing-mode doesn't match the line's writing-mode.
         LayoutUnit pointLogicalLeft = isHorizontalWritingMode() ? point.x() : point.y();
@@ skipping 1390 matching lines @@
 void LayoutBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const LayoutObject* obj) const
 {
     showLayoutObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace blink