Conformation box on Browser Close

Need to get a confirmation box when a browser gets closed …Anyone please help me on this …

I tried the below one but the Browser closes before any action is done

final Window win = new Window(windowName) {
protected void close() {
getWindow().showNotification(“Window closed by user”,Notification.TYPE_WARNING_MESSAGE);
Can some one help on how i can resolve it … or can suggest me an alternate solution for this .

Lemme knw if you get a solution for this

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