Function read_display_geometry 

fn read_display_geometry<R: Runtime>(
    app: &AppHandle<R>,
) -> Option<(Vec<Monitor>, Option<usize>)>

Read the display layout on the main thread and hand back plain data.

Every caller of fire_break runs on a tokio worker (the scheduler run loop, the IPC handler, and the tray menu all go through async_runtime::spawn), which is exactly the thread the monitor getters must not be called from — see crate::display::on_main_thread for the mechanism, the create_webview exclusion that keeps ensure_overlay off the main thread, and the degrade-to-None behaviour (here: no overlay this time, rather than risking a hang).

Both reads share a single hop so the returned monitor list and primary come from one consistent snapshot. Resolving the primary’s index happens inside the hop too, so the caller receives only plain data and never re-touches a live handle.