In the scope of bug 1846601 we introduced the "browsingContext.navigationFailed" event, but at the moment of the implementation there was no way to get the url which a user intended to navigate when the navigation was interrupted by beforeunload prompt. So we need first to get a platform support for it (see bug 1908951).
Bug 1908952 Comment 0 Edit History
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In the scope of bug 1846601 we introduced the "browsingContext.navigationFailed" and "browsingContext.navigationStarted" events in case of beforeunload prompt to be opened, but at the moment of the implementation there was no way to get the url which a user intended to navigate when the navigation was interrupted by beforeunload prompt. So we need first to get a platform support for it (see bug 1908951).