This class is used to aid in keeping track of DisplayChangedListeners and notifying them when a display change has taken place. DisplayChangedListeners are notified when the display's bit depth is changed, or when a top-level window has been dragged onto another screen. It is safe for a DisplayChangedListener to be added while the list is being iterated. The displayChanged() call is propagated after some occurrence (either due to user action or some other application) causes the display mode (e.g., depth or resolution) to change. All heavyweight components need to know when this happens because they need to create new surfaceData objects based on the new depth. displayChanged() is also called on Windows when they are moved from one screen to another on a system equipped with multiple displays.