src/Pure/PIDE/document.scala

       source: String,
       chunk: Symbol.Text_Chunk,
       changed: Boolean
     ) {
       def source_wellformed: Boolean = bytes.wellformed_text.nonEmpty
-      def unchanged: Item = copy(changed = false)
+      def unchanged: Item = if (changed) copy(changed = false) else this
     }
 
     def apply(blobs: Map[Node.Name, Item]): Blobs = new Blobs(blobs)
     val empty: Blobs = apply(Map.empty)