LoadingFragment | implement Fragment

As per the considerations for child and sibling fragments guide:

Only one FragmentManager is allowed to control the fragment back stack at any given time. If your app shows multiple sibling fragments on the screen at the same time, or if your app uses child fragments, then one FragmentManager must be designated to handle your app's primary navigation.

To define the primary navigation fragment inside of a fragment transaction, call the setPrimaryNavigationFragment() method on the transaction, passing in the instance of the fragment whose childFragmentManager should have primary control.

As explained in the Interact programmatically with Navigation, the app:defaultNavHost="true" is calling setPrimaryNavigationFragment() for you on the NavHostFragment itself, but you aren't calling setPrimaryNavigationFragment() on the parent fragment - your RootSlideshowFragment. This means that none of the logic to automatically handle the back button correctly is being called - you've essentially broke the chain from the activity to the parent fragment to the NavHostFragment. This causes you to intercept the back button even when there's no back stack (Navigation doesn't actually remove the last fragment when you pop it as otherwise it would not be present when your activity's exit animation is running).

Therefore to fix this, you need to:

1. Remove your OnBackPressedCallback from RootSlideshowFragment. It is not needed.
2. When you swap between fragments in your MainActivity, call setPrimaryNavigationFragment() on the new Fragment: