Inspector: should warn also when page is closed

[neos-bot]

Jira issue originally created by user @liwo:

The inspector should show a warning if the window is closed if there's still unsaved data.

Jira-URL: https://jira.neos.io/browse/NEOS-534

[mhsdesign]

This would to the trick: (in combination with some checks if there are unsaved (unapplied) changed)

    public warnUserForPossibleNotSavedChanges(e: Event) {
        const confirmationMessage = `Es könnte sein, dass die letzten Änderungen noch nicht gespeichert sind.`;
        // @ts-expect-error
        (e || window.event).returnValue = confirmationMessage;
        return confirmationMessage;
    }

    public componentDidMount() {
        window.addEventListener(
            "beforeunload",
            this.warnUserForPossibleNotSavedChanges
        );
    }

    public componentWillUnmount() {
        window.removeEventListener(
            "beforeunload",
            this.warnUserForPossibleNotSavedChanges
        );
    }

[mhsdesign]

duplicate of #2670 (comment)