IssueTracker

I'd like to add that this automatic removal of the NavGraph at the top of the stack is also making it hard for me to implement proper lateral top-level navigation.
A practical case where this kind of navigation happens is with a Material Design bottom navigation bar, where I have a NavHost above the bottom bar.

Given this navigation scheme:
A - B   (top-level)
     B1 (first sub-level)

When starting the Activity, the back stack is
0. A
1. NavGraph

If I wanted to navigate to B laterally, I would do:
navController.popBackStack(R.id.navGraph, false)
navController.navigate(R.id.B)

But the resulting back stack is:
0. B

Now, navigating between top-level destinations actually works (though popBackStack returns false); the problem arises again if I navigate to B1:
0. B1
1. B

When navigating from B1 to a top-level destination like A, the stack becomes:
0. A
1. B1
2. B

That is, A is not a top-level destination in the back stack: if I press back from A, I intend to exit the app, but I'm taken back to B1.

It would be nice to have a fixed reference to the bottom of the stack. Right now I'm working around this with a horrible ` while (navController.popBackStack()) continue`.

Is this going to be fixed?

I have the same issue with a setup as follows:

Navigation graph.

    <navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        app:startDestination="@+id/startupGraph">

        <navigation
            android:id="@+id/startupGraph"
            app:startDestination="@id/launcher_home">
            <fragment
                android:id="@+id/launcher_home"
                android:name="com.example.android.codelabs.navigation.MainFragment"
                android:label="@string/home"
                tools:layout="@layout/main_fragment" />
        </navigation>

        <navigation
            android:id="@+id/homeGraph"
            app:startDestination="@id/flow_step_one">
            <fragment
                android:id="@+id/flow_step_one"
                android:name="com.example.android.codelabs.navigation.FlowStepOneFragment"
                tools:layout="@layout/flow_step_one_fragment" />
            <fragment
                android:id="@+id/flow_step_two"
                android:name="com.example.android.codelabs.navigation.FlowStepTwoFragment"
                tools:layout="@layout/flow_step_two_fragment" />
        </navigation>

    </navigation>
   
MainFragment.

        override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
            super.onViewCreated(view, savedInstanceState)
             view.findViewById<Button>(R.id.navigate_dest_bt).setOnClickListener{
                 Navigation.findNavController(it)
                         .navigate(R.id.homeGraph, null, NavOptions.Builder()
                                 .setPopUpTo(R.id.startupGraph, false)
                                 .build())
             }
        }

FlowStepOneFragment.

        view.findViewById<View>(R.id.next_button).setOnClickListener {
            Navigation.findNavController(it)
                    .navigate(R.id.flow_step_two, null, NavOptions.Builder()
                            .setPopUpTo(R.id.flow_step_one, true)
                            .build())
        }
    }
}

Causes the following error:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.android.codelabs.navigation, PID: 12579
    java.lang.IllegalArgumentException: Navigator androidx.navigation.fragment.FragmentNavigator@d70ec6f reported navigation to unknown destination id com.example.android.codelabs.navigation:id/flow_step_two