Use case:
Would like to use an existing app layout and take benefit of offscreen screen "caching"

The issue
Currently if I attempt to use NavHostFragment in combination with ViewPager2 and FragmentStateAdapter I'll get the following runtime exception due to uninitialised containterViewId (stack trace below).

If I change FragmentStateAdapter.placeFragmentInViewHolder
From:

// { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
if (!shouldDelayFragmentTransactions()) {
scheduleViewAttach(fragment, container);
mFragmentManager.beginTransaction().add(fragment, "f" + holder.getItemId()).commitNow();
}

To:

// { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
if (!shouldDelayFragmentTransactions()) {
mFragmentManager.beginTransaction().add(container.getId(), fragment, "f" + holder.getItemId()).commitNow();
}

The issue is solved. Testing a couple of scenarios did not reveal any side effects, but as I have yet to familiarise myself with the details of the implementation I'm not sure if it will not have any negative side effects.

Happy to put up a PR for this, but would like to understand better why the fragment is being added this way in the current state of the adapter.

Stack trace:

java.lang.IllegalArgumentException: Must use non-zero containerViewId
at androidx.fragment.app.FragmentTransaction.replace(FragmentTransaction.java:223)
at androidx.fragment.app.FragmentTransaction.replace(FragmentTransaction.java:200)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:181)
at androidx.navigation.fragment.FragmentNavigator.navigate(FragmentNavigator.java:58)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:80)
at androidx.navigation.NavGraphNavigator.navigate(NavGraphNavigator.java:30)
at androidx.navigation.NavController.navigate(NavController.java:877)
at androidx.navigation.NavController.onGraphCreated(NavController.java:524)
at androidx.navigation.NavController.setGraph(NavController.java:482)
at androidx.navigation.NavController.setGraph(NavController.java:447)
at androidx.navigation.fragment.NavHostFragment.onCreate(NavHostFragment.java:248)
at androidx.fragment.app.Fragment.performCreate(Fragment.java:2613)
at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:831)
at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1228)
at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1293)
at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2066)
at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1856)
at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1811)
at androidx.fragment.app.FragmentManagerImpl.execSingleAction(FragmentManagerImpl.java:1686)
at androidx.fragment.app.BackStackRecord.commitNow(BackStackRecord.java:293)
at androidx.viewpager2.adapter.FragmentStateAdapter.placeFragmentInViewHolder(FragmentStateAdapter.java:284)
at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:222)
at androidx.viewpager2.adapter.FragmentStateAdapter.onViewAttachedToWindow(FragmentStateAdapter.java:61)
at androidx.recyclerview.widget.RecyclerView.dispatchChildAttached(RecyclerView.java:7433)
at androidx.recyclerview.widget.RecyclerView$5.addView(RecyclerView.java:862)
at androidx.recyclerview.widget.ChildHelper.addView(ChildHelper.java:107)
at androidx.recyclerview.widget.RecyclerView$LayoutManager.addViewInt(RecyclerView.java:8478)
at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(RecyclerView.java:8436)
at androidx.recyclerview.widget.RecyclerView$LayoutManager.addView(RecyclerView.java:8424)
at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1641)
at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1587)
at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:665)
at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:4040)
at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3757)
at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4310)