src/Pure/PIDE/document_editor.scala

       GUI_Thread.later { delay.revoke(); update(text) }
     }
 
     GUI_Thread.later { update() }
   }
+
+
+  /* global state */
+
+  sealed case class State(
+    session_info: Option[Sessions.Info] = None,
+    views: Set[AnyRef] = Set.empty,
+  ) {
+    def is_active: Boolean = session_info.isDefined && views.nonEmpty
+
+    def register_view(id: AnyRef): State = copy(views = views + id)
+    def unregister_view(id: AnyRef): State = copy(views = views - id)
+  }
 }