Challenge 3: popUpTo Behavior — Solution

// res/navigation/nav_graph.xml — add homeFragment and the popUpTo action
<fragment
    android:id="@+id/detailsFragment"
    android:name=".DetailsFragment"
    android:label="Details">
    <action
        android:id="@+id/action_details_to_home"
        app:destination="@id/homeFragment"
        app:popUpTo="@id/welcomeFragment"
        app:popUpToInclusive="true" />
</fragment>

<fragment
    android:id="@+id/homeFragment"
    android:name=".HomeFragment"
    android:label="Home" />

// DetailsFragment.kt — navigating using the new action
binding.goHomeButton.setOnClickListener {
    findNavController().navigate(R.id.action_details_to_home)
}

// What pressing Back from homeFragment does as a result:
// popUpTo="@id/welcomeFragment" with popUpToInclusive="true" removes
// welcomeFragment AND everything above it (which includes detailsFragment,
// since it's above welcomeFragment on the stack) from the back stack at
// the moment of navigating to homeFragment. After this navigation, the
// back stack contains ONLY homeFragment — neither welcomeFragment nor
// detailsFragment is still on it. So pressing Back from homeFragment
// does NOT return to detailsFragment or welcomeFragment at all — it
// exits the app (or returns to whatever launched it), exactly like a
// "you can't go back to the login/intro flow" pattern.

Notes:
- popUpToInclusive="true" is what makes welcomeFragment itself get
  removed too — without it (popUpToInclusive="false", the default),
  welcomeFragment would remain on the stack as the new "bottom," and
  Back from homeFragment would return there instead of exiting.
- This is the same underlying mechanism used for a real login flow (per
  the chapter's example): once past the destinations meant to be
  one-time (an intro sequence, a login screen), popUpTo removes them so
  Back can't accidentally re-trigger that flow.