Note that when the window is moved, it might get partially exposed or partially hidden by other windows, and thus we might get Expose events due to this operation.

• 注意当窗口被移动的时候,它可能会被部分暴露或者部分被其他窗口遮盖,因而我们可能因为这个操作的缘故和收到暴露事件。