The event messenger.compose.onBeforeSend() does not lock the compose window completely. It is still possible to interact with the menu and the toolbar buttons.
For example: You can still change the delivery format or the priority. And you can even fiddle with the editor, i.e. the body of the message, via "Tools -> Quote Message". Multiple menu entries are still enabled, but seem to have no effect.
I think the compose window should get locked completely in this case - including locking all menus and toolbars.