Some questions about dynamic threadpool

[JingLiofLiJing]

In dynamic_worker_threadpool.cc file, i think WaitForNotify() are used to several scenarios like:
1.between push and pop idle_threads_stack, add_task() firstly pop the stack, which make pinfo !=info, and set() before wait(), so thread don't need to wait.
2.pinfo ==info, thread automatically loop for tasks.
3.when pinfo !=info, the set() make another thread wake and continue to work(by condition signal).
So i may have confused by 2 questions:
1.before shutdown(), it seems like each time at least one thread must be active, because each thread always waked from set() which set is_set to true, so when WaitForIdle() could return?
2.Shutdown() set stopped_ to false to make active thread breaking the loop to complete. Why need to maintain the one last thread to wait for event_for_all_workers_exit_ signal? I think that when thread_num_ decrease to zero, event_for_all_workers_exit_ wait() could be return directly.