RFE: Warn when closing while "Waiting for the server to respond"

On intermittent network connections, JetPunk will display the messages "Waiting for the server to respond." and "Error saving quiz results. You may not be connected to the internet." These messages are displayed at the top of the page and so may not be visible to the user (= me :-)) when closing the tab, thus not recording that a particular quiz was finished. :-(

Other web applications like Google Keep display an "are you sure?" warning when there are local changes that have not yet been synchronized with the server. They use the beforeunload event for that (cf. https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event). It would be nice if JetPunk could also use that, probably from the moment a quiz is started to when its "score" has successfully been recorded by the server.