Empty dialog box with window.onbeforeunload

When opening https://code-maven.com/try/examples/js/prevent_leaving_the_page.html and then entering another URL, Chromium displays a "Do you want to leave this site?" dialogbox.

QtWebEngine simply displays an empty confirmation box (without text), and this doesn't seem to be overridable. I think there should be a QWebEnginePage::javaScriptConfirmLeave(const QUrl securityOrigin, const QString &msg) or so for this, or at the very least, a better text should be displayed.