Bug 1420415 - TextEditor::CreateBRImpl() needs to make pointToInsertBrNode store mOffset before calling EditorBase::CreateNode() r=m_kato

When TextEditor::CreateBRImpl() splits a text node before inserting new <br>
element, it initializes pointToInsertBrNode only with the right text node.
Then, it refers its Offset() after inserting new <br> node before the point.
Therefore, the offset is computed with the new DOM tree.  So, adding 1 to
the offset is redundant only in this case.

So, before calling CreateNode(), it needs to make pointToInsertBrNode store
offset with calling its Offset().

Note that this ugly code will be replaced with patches for bug 1408227.
Additionally, this doesn't use AutoEditorDOMPointChildInvalidator because
it's not available in 58 but we need to uplift this patch.

Finally, I'm not sure how to check this in automated tests.  Therefore, this
patch doesn't include automated tests.

MozReview-Commit-ID: IaQBonoGawR

--HG--
extra : rebase_source : a89559932f27d98a02baf3e207c6be3c2a545aad

editor/libeditor/TextEditor.cpp

@@ -466,6 +466,9 @@ TextEditor::CreateBRImpl(nsCOMPtr<nsIDOMNode>* aInOutParent,
       pointToInsertBrNode.Set(node);
       Unused << newLeftNode;
     }
+    // Lock the offset of pointToInsertBrNode because it'll be referred after
+    // inserting a new <br> node before it.
+    Unused << pointToInsertBrNode.Offset();
     // create br
     brNode = CreateNode(nsGkAtoms::br, pointToInsertBrNode);
     if (NS_WARN_IF(!brNode)) {