I would like to query whether the IsDirty flag is true or false when I click on the back button, but it should be before and not when the navigation is already finished. I'm not sure if this is so easy or if it's a bit more complex.
There is no "Simple" answer to this problem. While you can use various strategies within page and navigation menus to restrict dirty form exits, there are no easy one-liner ways to control browser navigation requests.
My current solution to this problem, which addresses all normal form exit mechanisms can be found here Blazr.Demo.Routing.
It rebuilds Router, adds a new Navigation Manager and interfaces with the browser beforeunload event.