When new session is opened.

What event can I use to update my UI when an new session is open and all controls have been displayed?
Is Open the correct event?

No, definitely not the Open event. Use the Shown event.

How ‘typically’ does the App know that a new session has been ‘shown’?

The browser fires a Javascript event when the page has been received and rendered on the screen. I presume Xojo uses that event to report back to the app and fire the Shown event.

Right now I’ve put a call in my web page shown event that calls a method in the App.
There doesn’t seem to be any other way for the app to know that the sessions web page is shown.

I think I misunderstood what you were asking before. The Shown event is for this purpose. If you want to identify an brand new Session, then you should set a flag in the session to tell you that this is the first page being shown in this session.

No… I need the app to know when a session has been shown so that it is safe to start asking it to display things.
So as I said for now I have the session send call a method in the App to indicate that it is ready.