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成功启用全屏模式后，包含全屏元素的文档会收到 fullscreenchange 事件。退出全屏模式时，文档再次收到 fullscreenchange 事件。请注意， fullscreenchange 事件本身不会提供有关文档是进入还是退出全屏模式的任何信息，但是如果文档具有非null fullScreenElement ，你知道你处于全屏模式。

When full-screen mode is successfully engaged, the document which contains the full-screen element receives a fullscreenchange event. When full-screen mode is exited, the document again receives a fullscreenchange event. Note that the fullscreenchange event doesn't provide any information itself as to whether the document is entering or exiting full-screen mode, but if the document has a non null fullScreenElement , you know you're in full-screen mode.