DropDown: Toggling open state multiple times introduces memory leak and exceptions when scrolling document if control is disposed

[FlowIT-JIT]

Consider https://jsfiddle.net/65Lnt0h1/2/

Toggling the open state of the DropDown control multiple times causes multiple event registrations, of which only the most recent registration is cleaned up. This results in a memory leak, and causes exceptions to be thrown if the control is disposed and its scroll parent is scrolled.