Session.ConfirmMessage ignores string setting

I notice that the string I assign to Session.ConfirmMessage in Session.Open is ignored when running Mac Safari and Mac Chrome. I get a browser-specific generic message when attempting to close the browser tab or when hitting the Back button.

Xojo 2018r4
macOS 10.13.16

The docs say that Firefox emits a generic message:

“On Firefox 4, a generic message is used in place of the specific text specified here.”

Maybe Safari and Chrome are following this practice as well?