The event is Wisej.Web.ToolBar.ButtonDropDown. All child component (not directly in designable, like a TreeNode for example) fire their events on the parent container.
We have extended the TreeNode class and the ToolBarButton class (and others) to replicate the event at the class level to allow for easy class extension. But the ToolBarButton class only reflects the Click event. Will probably enhance it to reflect ButtonDropDown as well.