Steps to Reproduce

run attached sample (zul+java, click button

Current Result

event loops infinitely, never resolves

Expected Result

exception in event shouldn't create an infinite loop

Debug Information

org.zkoss.zk.ui.impl.DesktopImpl.ScheduleListener

if (ifPresent != null) {
    ifPresent.invoke();
    _schedInfos.invalidate(key);

invoke is called before removing the schedule info from the cache.
If an exception occurs, invalidate is never called, which causes the item to not be removed from the _schedInfos

Workaround

attached replacement DesktopImpl for 9.6.4
deploy to src/main/java/org/zkoss/zk/ui/impl/DesktopImpl.java
to override original class