changes for sub-level threading

mailnews/base/public/nsIMsgThread.idl

@@ -38,7 +38,7 @@ interface nsIMsgThread : nsISupports {
   nsIMsgDBHdr GetChildAt(in long index);
   nsIMsgDBHdr GetChild(in nsMsgKey msgKey);
   nsIMsgDBHdr GetChildHdrAt(in long index);
-
+  nsIMsgDBHdr GetRootHdr(out long index);
   void RemoveChildAt(in long index);
   void RemoveChildHdr(in nsIMsgDBHdr child);
 

mailnews/base/src/nsMessageViewDataSource.cpp

@@ -718,7 +718,7 @@ PRBool nsMessageViewDataSource::IsThreadsFirstMessage(nsIMsgThread *thread, nsIM
 	nsCOMPtr<nsIMsgDBHdr> firstHdr;
 	nsresult rv;
 
-	rv = thread->GetChildHdrAt(0, getter_AddRefs(firstHdr));
+	rv = thread->GetRootHdr(nsnull, getter_AddRefs(firstHdr));
 	if(NS_FAILED(rv))
 		return PR_FALSE;