currently _schedInfos in DesktopImpl is manually synchronized

https://github.com/zkoss/zk/blob/v8.6.0.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L163-L164

Using a ConcurrentLinkedQueue (since java 1.5) can avoid synchronized blocks when adding ScheduleListeners to the end (.offer()) ...
https://github.com/zkoss/zk/blob/v8.6.0.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L1597-L1603

and removing them from the start of the queue (.poll()).
https://github.com/zkoss/zk/blob/v8.6.0.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L1802-L1825

At the same time the loop code can be simplified (no need to copy the contents, or re-add intermediately added items)