handleTooltip(event, item, show) {
      if (item && item.tooltip != null) {
        item = resolveItem(item, event, this.canvas(), this._origin);
        const value = show && item && item.tooltip || null;
        this._tooltip.call(this._obj, this, event, item, value);
      }
    }
Handle tooltip display in response to an item.tooltip value.
@param {Event} event - The event triggering tooltip display.
@param {Item} item - The scenegraph item.
@param {boolean} show - A boolean flag indicating whether
  to show or hide a tooltip for the given item.