The org.zkoss.zk.ui.event.EventThreadSuspend interface
From Documentation
EventThreadSuspend is implemented by a listener class that will be called before an event processing thread is going to be suspended.
If a listener implements this interface, an instance is created, and then the beforeSuspend method, when an event processing thread is going to suspended. It executes in the event processing thread.
A developer can prevent can prevent an event processing thread from being suspended by throwing an exception.
A typical use of this feature is to limit the number of suspended threads.