EXTENSION HELPER FUNCTIONS
To provide support for the moreup, moredown, moreleft, and moreright properties in an element:
First the extension writer must set up the required properties. Extension writers can do this themselves, or can call the InitialiseMoreUpDownProps() and InitialiseMoreLeftRightProps() helper functions.
Next, extension writers are then responsible for adding a listener to the viewmodel model of the element's attached widget, and then calling ApplyScrollProps() with any ModelEvents that are returned from the model.