You go from the navigation app to the new planner, and then from the planner to the forum. Once you're on the forum, there's no button to take you back to the navigation app. As NN = Navigation Next. Have I understood you correctly?

But surely you don't need a button for that in the forum? If you ended up on the forum from the navigation app, you can simply use your device's operating system to switch back to the navigation app, can't you?

On my iPhone, I just swipe to the right at the bottom of the screen and the previous app becomes active again. Doesn't Android have something similar, either by swiping or using the three vertical stripes at the bottom of the screen to switch back to another open app?