canMergeLists() should make sure that the list items do not belong to the same list

Source/core/editing/DeleteSelectionCommand.cpp

 /*
  * Copyright (C) 2005 Apple Computer, 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 617 matching lines @@
 
     VisiblePosition endOfParagraphToMove = endOfParagraph(startOfParagraphToMove, CanSkipOverEditingBoundary);
 
     if (mergeDestination == endOfParagraphToMove)
         return;
 
     // If the merge destination and source to be moved are both list items of different lists, merge them into single list.
     Node* listItemInFirstParagraph = enclosingNodeOfType(m_upstreamStart, isListItem);
     Node* listItemInSecondParagraph = enclosingNodeOfType(m_downstreamEnd, isListItem);
     if (listItemInFirstParagraph && listItemInSecondParagraph
+        && listItemInFirstParagraph->parentElement() != listItemInSecondParagraph->parentElement()
         && canMergeLists(listItemInFirstParagraph->parentElement(), listItemInSecondParagraph->parentElement())) {
         mergeIdenticalElements(listItemInFirstParagraph->parentElement(), listItemInSecondParagraph->parentElement());
         m_endingPosition = mergeDestination.deepEquivalent();
         return;
     }
 
     // The rule for merging into an empty block is: only do so if its farther to the right.
     // FIXME: Consider RTL.
     if (!m_startsAtEmptyLine && isStartOfParagraph(mergeDestination) && startOfParagraphToMove.absoluteCaretBounds().x() > mergeDestination.absoluteCaretBounds().x()) {
         if (isHTMLBRElement(*mergeDestination.deepEquivalent().downstream().deprecatedNode())) {
@@ skipping 209 matching lines @@
 
 // Normally deletion doesn't preserve the typing style that was present before it.  For example,
 // type a character, Bold, then delete the character and start typing.  The Bold typing style shouldn't
 // stick around.  Deletion should preserve a typing style that *it* sets, however.
 bool DeleteSelectionCommand::preservesTypingStyle() const
 {
     return m_typingStyle;
 }
 
 } // namespace WebCore