Variable to know if back layer is expanded or collapsed

I am creating a mechanism where the back layer displays a list of items, and the front layer shows detailed information about the selected item.

Once an item is selected from the back layer, I want the appbar title to display the name of the selected item, else if the back layer is expanded, I want to set the appbar title to something more generic such as "Items List".

Is there a way to do this natively with backdrop? Or is there a variable that can be checked so that I can at least know the current state of the back layer (either collapsed or expanded)?