I've looked around in the available properties from a tag binding expression, in Session and Page and View properties and can't seem to find anything that would indicate weather the page is in full screen mode. I would assume it would be in Page properties if my understanding is correct.
I am trying to make a simple label in my header that will say "Press F11 for full screen mode" if it is currently not in full screen mode.
Although, now that I think about it, it might be better to make a button to do it if a user is on a tablet or phone. Still, I would like to change the symbol for the button like a typical video full screen toggle, which still utilizes knowing the current state of full screen mode.
Unfortunately, I think you'd be forced to do your own tracking here. I can't think of any property which exposes this within Perspective. What makes this even more difficult is that the Fullscreen Action has no corresponding scripting function (because it's "not possible"), so you'd have to have a Fullscreen Action and a sibling Scripting Action to update the session state. But... The Action can't be deterministic...
I think the way I would handle this is to have two different Views: one that has just a button which has the Fullscreen Action configured to set the Session to be in fullscreen mode and a sibling Script Action which updates some session property to reflect the change in state, and a second View which has the same pieces but which turns off fullscreen.
Then you can place an Embedded View component and bind the EmbeddedView.props.viewPath property to the custom session property which reflects the session state.