一个空闲的客户机程序是指：在某一确定时间内，该客户程序没有传输键盘或指针事件。
An idle client is one which has transmitted no keyboard or pointer events for more than a certain length of time.
当循环指针移动到某个位置，该位置所对应的链表上的所有定时器超时。
When a circular pointer moved to a certain position, all timers in the corresponding link had already expired.
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