code: function getPanelByCover(controller, cover) {
      var panels = controller._panels;
      if (!panels) {
          return true; // Global panel
      }
      var panelId = cover.__brushOption.panelId;
      // User may give cover without coord sys info,
      // which is then treated as global panel.
      return panelId != null ? panels[panelId] : true;
  }
docs: Return a panel or true