The notification arrives. Sometimes the app shows it, sometimes the system does, sometimes tapping it opens the right screen and sometimes it dumps the user on the home page. The behaviour feels random until you see the structure underneath: the same message takes four different paths depending on what the app was doing. App state Who displays it Which handler runs Foreground Nobody, by default onMessage Background The OS onBackgroundMessage (separate isolate) Terminated The OS onBackgroundMessage (separate isolate) Opened by tapping — onMessageOpenedApp , or getInitialMessage if it was terminated Every notification bug I have debugged was one of those rows being unhandled. Wire all four and most of the mystery disappears. Notification messages versus data messages Before the code, the distinction that determines everything: an FCM payload can contain a notification block, a data block, or both. notification present — the OS displays it automa...