fix next unread going to dummy header in grouped view, sr=mscott 289454

mailnews/base/src/nsMsgGroupView.cpp

@@ -296,7 +296,7 @@ nsMsgGroupThread *nsMsgGroupView::AddHdrToThread(nsIMsgDBHdr *msgHdr, PRBool *pN
     if (GroupViewUsesDummyRow())
     {
       foundThread->m_dummy = PR_TRUE;
-      msgFlags |=  MSG_VIEW_FLAG_DUMMY | MSG_VIEW_FLAG_HASCHILDREN;
+      msgFlags |=  MSG_VIEW_FLAG_DUMMY | MSG_VIEW_FLAG_HASCHILDREN | MSG_FLAG_READ;
     }
 
     nsMsgViewIndex insertIndex = GetInsertIndex(msgHdr);