The OnClientSplitButtonClicking client-side event occurs when the user clicks on a ribbonbar split button, before the ribbonbar responds to the mouse click.
The event handler receives two parameters:
The instance of the ribbonbar firing the event.
An eventArgs parameter containing the following methods:
get_button() returns a reference to the ribbonbar split button that was clicked. In this case it is RibbonBarSplitButton.
set_cancel() lets you prevent the ribbonbar from responding to the mouse click.