A listener to be registered upon the combo box (
not its popup menu) to handle mouse events that affect the state of the popup menu. The main purpose of this listener is to make the popup menu appear and disappear. This listener also helps with click-and-drag scenarios by setting the selection if the mouse was released over the list during a drag.
Warning: We recommend that you not create subclasses of this class. If you absolutely must create a subclass, be sure to invoke the superclass version of each method.