Function optimistic_push_screen_stack 

pub fn optimistic_push_screen_stack(
    has_account: bool,
    screen_stack: Vec<Screen>,
    screen: Screen,
) -> Option<Vec<Screen>>

Predicts the screen stack that [AppCore::push_screen] will produce, so a shell can show the destination before the core confirms it. None means the push has no predictable result and the shell must wait for the core.