window is considered mapped if an XMapWindow() call has been made on it. It may not be visible on the screen for one of the following reasons:
如果对窗口进行了XMapWindow（）调用，则认为该窗口已映射，窗口在屏幕上不显示有如下愿意：
- It is obscured by another opaque window.
- One of its ancestors is not mapped.
- It is entirely clipped by an ancestor.
- 它被一个看不到的透明窗口遮挡了。
- 一个祖先窗口没有被映射。（可能是父窗口，也可以能是父窗口的父窗口）
- 它完全被祖先剪掉了。（clipped 翻译裁剪，具体啥意思求教。）
Expose events are generated for the window when part or all of it becomes visible on the screen. A client receives the Expose events only if it has asked for them. Windows retain their position in the stacking order when they are unmapped.