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sousa, after search on the google with key word "cancel beforeunload", I found, the return value of a function which register to beforeunload must be a String, this stirng is for displaying a confirm message to user by Browser. you have no way to cancel it, and also no possible to replace with your logic.(showMsg in your code) So It is not possible to sent a event back to event to server if user decide to close/refresh the tab.

This is browser restriction...