static void OnTooltip(TooltipEvent e, string tooltip)
        {
            VisualElement element = e.currentTarget as VisualElement;

            if (element != null)
            {
                e.rect = element.worldBound;
            }
            e.tooltip = tooltip;
            e.StopImmediatePropagation();
        }
        static void OnTooltip(TooltipEvent e)
        {
            VisualElement element = e.currentTarget as VisualElement;

            if (element != null && !string.IsNullOrEmpty(element.tooltip))
            {
                e.rect    = element.worldBound;
                e.tooltip = element.tooltip;
                e.StopImmediatePropagation();
            }
        }