diff -r 000000000000 -r 6474c204b198 editor/libeditor/base/JoinElementTxn.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/editor/libeditor/base/JoinElementTxn.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,122 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <stdio.h>                      // for printf
+
+#include "JoinElementTxn.h"
+#include "nsAString.h"
+#include "nsDebug.h"                    // for NS_ASSERTION, etc
+#include "nsEditor.h"                   // for nsEditor
+#include "nsError.h"                    // for NS_ERROR_NULL_POINTER, etc
+#include "nsIDOMCharacterData.h"        // for nsIDOMCharacterData
+#include "nsIEditor.h"                  // for nsEditor::IsModifiableNode
+#include "nsINode.h"                    // for nsINode
+#include "nsISupportsImpl.h"            // for EditTxn::QueryInterface, etc
+
+JoinElementTxn::JoinElementTxn()
+  : EditTxn()
+{
+}
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(JoinElementTxn, EditTxn,
+                                   mLeftNode,
+                                   mRightNode,
+                                   mParent)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(JoinElementTxn)
+NS_INTERFACE_MAP_END_INHERITING(EditTxn)
+
+NS_IMETHODIMP JoinElementTxn::Init(nsEditor   *aEditor,
+                                   nsIDOMNode *aLeftNode,
+                                   nsIDOMNode *aRightNode)
+{
+  NS_PRECONDITION((aEditor && aLeftNode && aRightNode), "null arg");
+  if (!aEditor || !aLeftNode || !aRightNode) { return NS_ERROR_NULL_POINTER; }
+  mEditor = aEditor;
+  mLeftNode = do_QueryInterface(aLeftNode);
+  nsCOMPtr<nsIDOMNode>leftParent;
+  nsresult result = mLeftNode->GetParentNode(getter_AddRefs(leftParent));
+  NS_ENSURE_SUCCESS(result, result);
+  if (!mEditor->IsModifiableNode(leftParent)) {
+    return NS_ERROR_FAILURE;
+  }
+  mRightNode = do_QueryInterface(aRightNode);
+  mOffset = 0;
+  return NS_OK;
+}
+
+// After DoTransaction() and RedoTransaction(), the left node is removed from the content tree and right node remains.
+NS_IMETHODIMP JoinElementTxn::DoTransaction(void)
+{
+  NS_PRECONDITION((mEditor && mLeftNode && mRightNode), "null arg");
+  if (!mEditor || !mLeftNode || !mRightNode) { return NS_ERROR_NOT_INITIALIZED; }
+
+  // get the parent node
+  nsCOMPtr<nsIDOMNode> leftParent;
+  mLeftNode->GetParentNode(getter_AddRefs(leftParent));
+  NS_ENSURE_TRUE(leftParent, NS_ERROR_NULL_POINTER);
+
+  // verify that mLeftNode and mRightNode have the same parent
+  nsCOMPtr<nsIDOMNode> rightParent;
+  mRightNode->GetParentNode(getter_AddRefs(rightParent));
+  NS_ENSURE_TRUE(rightParent, NS_ERROR_NULL_POINTER);
+
+  if (leftParent != rightParent) {
+    NS_ASSERTION(false, "2 nodes do not have same parent");
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  // set this instance mParent. 
+  // Other methods will see a non-null mParent and know all is well
+  mParent = leftParent;
+  nsCOMPtr<nsINode> leftNode = do_QueryInterface(mLeftNode);
+  nsCOMPtr<nsINode> rightNode = do_QueryInterface(mRightNode);
+  nsCOMPtr<nsINode> parent = do_QueryInterface(mParent);
+  NS_ENSURE_TRUE(leftNode && rightNode && parent, NS_ERROR_FAILURE);
+  mOffset = leftNode->Length();
+
+  return mEditor->JoinNodesImpl(rightNode, leftNode, parent);
+}
+
+//XXX: what if instead of split, we just deleted the unneeded children of mRight
+//     and re-inserted mLeft?
+NS_IMETHODIMP JoinElementTxn::UndoTransaction(void)
+{
+  NS_ASSERTION(mRightNode && mLeftNode && mParent, "bad state");
+  if (!mRightNode || !mLeftNode || !mParent) { return NS_ERROR_NOT_INITIALIZED; }
+  nsresult result;
+  nsCOMPtr<nsIDOMNode>resultNode;
+  // first, massage the existing node so it is in its post-split state
+  nsCOMPtr<nsIDOMCharacterData>rightNodeAsText = do_QueryInterface(mRightNode);
+  if (rightNodeAsText)
+  {
+    result = rightNodeAsText->DeleteData(0, mOffset);
+  }
+  else
+  {
+    nsCOMPtr<nsIDOMNode>child;
+    result = mRightNode->GetFirstChild(getter_AddRefs(child));
+    nsCOMPtr<nsIDOMNode>nextSibling;
+    uint32_t i;
+    for (i=0; i<mOffset; i++)
+    {
+      if (NS_FAILED(result)) {return result;}
+      if (!child) {return NS_ERROR_NULL_POINTER;}
+      child->GetNextSibling(getter_AddRefs(nextSibling));
+      result = mLeftNode->AppendChild(child, getter_AddRefs(resultNode));
+      child = do_QueryInterface(nextSibling);
+    }
+  }
+  // second, re-insert the left node into the tree 
+  result = mParent->InsertBefore(mLeftNode, mRightNode, getter_AddRefs(resultNode));
+  return result;
+
+}
+
+NS_IMETHODIMP JoinElementTxn::GetTxnDescription(nsAString& aString)
+{
+  aString.AssignLiteral("JoinElementTxn");
+  return NS_OK;
+}