src/Pure/PIDE/document.scala

             val name = command.node_name.node
             val sources_iterator =
               node.commands.iterator.takeWhile(_ != command).map(_.source) ++
                 (if (offset == 0) Iterator.empty
                  else Iterator.single(command.source(Text.Range(0, command.chunk.decode(offset)))))
-            val pos = (Line.Position.zero /: sources_iterator)(_.advance(_))
+            val pos = (Line.Position.zero /: sources_iterator)(_.advance(_, Length))
             Line.Node_Position(name, pos)
           }
 
 
         /* cumulate markup */