if (cm.display.viewFrom != cm.display.reportedViewFrom || cm.display.viewTo != cm.display.reportedViewTo) {
    update.signal(cm, "viewportChange", cm, cm.display.viewFrom, cm.display.viewTo)
    cm.display.reportedViewFrom = cm.display.viewFrom; cm.display.reportedViewTo = cm.display.viewTo
  }