src/Tools/jEdit/src/document_model.scala

   def node_header(): Document.Node.Header =
   {
     Swing_Thread.require()
     Isabelle.buffer_lock(buffer) {
       Exn.capture {
-        Isabelle.thy_load.check_thy_text(name, buffer.getSegment(0, buffer.getLength))
+        val text = buffer.getSegment(0, buffer.getLength)
+        Isabelle.thy_load.check_thy_text(session.recent_syntax, name, text)
       } match {
         case Exn.Res(header) => header
         case Exn.Exn(exn) => Document.Node.bad_header(Exn.message(exn))
       }
     }