Disallow relayout roots inside multicol.

Source/core/layout/LayoutObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 724 matching lines @@
     if (!object->hasOverflowClip())
         return false;
 
     if (object->style()->width().isIntrinsicOrAuto() || object->style()->height().isIntrinsicOrAuto() || object->style()->height().hasPercent())
         return false;
 
     // Table parts can't be relayout roots since the table is responsible for layouting all the parts.
     if (object->isTablePart())
         return false;
 
+    // Inside multicol it's generally problematic to allow relayout roots. The multicol container
+    // itself may be scheduled for relayout as well (due to other changes that may have happened
+    // since the previous layout pass), which might affect the column heights, which may affect how
+    // this object breaks across columns). Spanners may also have been added or removed since the
+    // previous layout pass, which is just another way of affecting the column heights (and the
+    // number of rows). Instead of identifying cases where it's safe to allow relayout roots, just
+    // disallow them inside multicol.
+    if (object->isInsideFlowThread())
+        return false;
+
     return true;
 }
 
 void LayoutObject::markContainerChainForLayout(bool scheduleRelayout, SubtreeLayoutScope* layouter)
 {
     ASSERT(!isSetNeedsLayoutForbidden());
     ASSERT(!layouter || this != layouter->root());
 
     LayoutObject* object = container();
     LayoutObject* last = this;
@@ skipping 2612 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         fprintf(stderr, "Cannot showLayoutTree. Root is (nil)\n");
     }
 }
 
 #endif