When an app is in a thumbnail state, it can be backgrounded further and become invisible. Your code must handle this transition from from the thumbnail state to the invisible state.
Backgrounding means the user taps the app, which is already in a thumbnail state, to an invisible state. The invisible state means that the app is not visible on the screen. The user may swipe sideways to hide the thumbnail state or may bring another app to the foreground, causing the screen to display the new app.
When the app is backgrounded, the following events are sent:
When the app is brought back to a thumbnail state, the following events are sent:
The following diagram shows the events that are received when the app is
backgrounded/unbackgrounded: 
Your code needs to handle the reception of each of the above events and properly respond to each of them. For more information regarding each of these events, see Various States and BPS Navigator Events sections.