Posted when an NSPopUpButton object receives a mouse-down event—that is, when the user is about to select an item from the menu.
The notification object is the selected NSPopUpButton object. This notification does not contain a userInfo dictionary.
Available in Mac OS X v10.0 and later.
Declaration Syntax Version Information- Available in Monobjc Bridge: 10.6 (For Mac OS X 10.6 and later), 10.5 (For Mac OS X 10.5 and later)