The OnClientSelectedTabChanging client-side event occurs when the user clicks on a ribbonbar tab, 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_tab() returns a reference to the ribbonbar tab which is about to be selected.
get_previouslySelectedTab() returns a reference to the currently selected tab.
get_domEvent() returns the DOM event object.
set_cancel() lets you cancel the event and prevent changing the selected tab.