The main dispatch queue is a serial queue and is a shared resource. If we
submit a long-running task to it, then no other tasks, including those submitted
by the system frameworks, can run until it completes.
Some events get queued for all GUI-connected threads but are only processed
by the first to dequeue them. Other threads which tend their event queue
discard such already-processed events. However, some threads may be
connected to the GUI but never tend their event queue. To prevent such
threads from accumulating zombie events, the zombies are cleared each time a
new event is queued.
Some events are application-wide, not specific to a thread. Such an event
needs to be broadcast to all GUI-attached threads because we don't know which
are handling events, but we don't want the event to be processed in each.
Often it should only be processed by the first to pull it from its queue.
Ken Thomases