Sample app demonstrating up navigation issues with nested fragments and the navigation components library

This app consists of one Activity and three main Fragments:

1. MainFragment
2. OnboardingFragment
3. GameFragment

The main navigation graph (global_nav_graph.xml) looks like this:

MainFragment -> OnboardingFragment -> GameFragment

The OnboardingFragment has two nested Fragments: OnboardingFragmentStep1Fragment and OnboardingFragmentStep2Fragment

However there are two problem with that:

1. Navigating up from OnboardingFragmentStep2

Likely caused by setting app:defaultNavHost="true" which in turn sets the nested nav host as the primary navigation Fragment on the OnboardingFragment's fragment manager.

That causes FragmentManager.popBackStackImmediate() to pop only the fragments in the nested host instead of the main host. The toolbar however is in the right state (title "main_activity" without up arrow).

2. Navigate from nested fragment to global navigation

In step 2 of the onboarding we want to give the user the possibility to advance to the GameFragment. However: the button for that resides in the nested NavHost which doesn't know about the GameFragment (see OnboardingFragmentStep2ViewModel).

So how should we issue the navigation from there?