How to fix onBackPressed for an expanded material3 SearchBar?

I am using androidx.compose.material3.SearchBar in its expanded state in my Android project. I am using androidx.compose:compose-bom:2024.09.02 which includes androidx.compose.material3:material3-*:1.3.0.

The screenshots shows what happens when a user performs the back gesture.

Here is the structure of the relevant classes and files:

SearchActivity.kt:

class SearchActivity {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_search)
        if (savedInstanceState == null) {
            addFragment(R.id.container, SearchFragment(), SearchFragment.FRAGMENT_TAG)
        }
    }
}

layout/activity_search.xml:

<androidx.constraintlayout.widget.ConstraintLayout
    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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:context=".search.SearchActivity"
        tools:ignore="MergeRootFrame"
        tools:layout="@layout/fragment_search" />

</androidx.constraintlayout.widget.ConstraintLayout>

SearchFragment.kt:

class SearchFragment : Fragment() {

    companion object {
        const val FRAGMENT_TAG = "SEARCH_FRAGMENT_TAG"

        fun replaceAtBackStack(fragmentManager: FragmentManager, @IdRes containerViewId: Int) {
            fragmentManager.commit {
                fragmentManager.popBackStack(FRAGMENT_TAG, FragmentManager.POP_BACK_STACK_INCLUSIVE)
                fragmentManager.replaceFragment(containerViewId, SearchFragment(), FRAGMENT_TAG, FRAGMENT_TAG)
            }
        }
    }

    private val viewModelFactory by lazy { SearchViewModelFactory(/*...*/) }
    private val viewModel: SearchViewModel by viewModels { viewModelFactory }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View = 
      inflater.inflate(R.layout.fragment_search, container, false).apply {
        findViewById<ComposeView>(R.id.search_view).apply {
            setViewCompositionStrategy(DisposeOnViewTreeLifecycleDestroyed)
            setContent {
                SearchScreen(
                    searchQuery = viewModel.searchQuery,
                    state = viewModel.searchResultsState.collectAsState().value,
                    onViewEvent = viewModel::onViewEvent,
                )
            }
            isClickable = true
        }
    }

}

layout/fragment_search.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.compose.ui.platform.ComposeView
        android:id="@+id/search_view"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

SearchScreen.kt:

@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun SearchScreen(
    searchQuery: String,
    state: SearchResultState,
    onViewEvent: (SearchViewEvent) -> Unit,
) {
    MyTheme {
        Scaffold { contentPadding ->
            Box(
                Modifier
                    .padding(contentPadding)
            ) {
                var text by rememberSaveable { mutableStateOf(searchQuery) }
                val expanded = true
                SearchBar(
                    modifier = Modifier
                        .align(TopCenter)
                        .semantics { traversalIndex = 0f },
                    inputField = {
                        SearchBarDefaults.InputField(
                            query = text,
                            onQueryChange = {
                                text = it
                                onViewEvent(OnSearchQueryChange(it))
                            },
                            onSearch = { },
                            expanded = expanded,
                            onExpandedChange = { },
                            placeholder = { Text(stringResource(R.string.search_query_hint)) },
                            leadingIcon = { Icon(Icons.Default.Search, contentDescription = null) },
                        )
                    },
                    expanded = expanded,
                    onExpandedChange = { }
                ) {
                    when (state) {
                        is Loading -> Loading()
                        is Success -> {
                            val sessions = state.parameters
                            if (sessions.isEmpty()) {
                                NoSearchResult()
                            } else {
                                SearchResultList(
                                    parameters = sessions,
                                    onViewEvent = onViewEvent,
                                )
                            }
                        }
                    }
                }
            }
        }
    }
}

For some reason, the SearchBar consumes the back gesture. - I noticed this when I replaced the SearchBar with some other trivial composable. - This prevents the user from leaving the screen at all which is what I expect to happen on the back gesture.
How can this be fixed?

I was missing a androidx.activity.compose.BackHandler which is available in androidx.activity:activity-compose. Once the dependency is added, you can add the BackHandler to the composable:

SearchScreen.kt:

@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun SearchScreen(
    searchQuery: String,
    state: SearchResultState,
    onViewEvent: (SearchViewEvent) -> Unit,
) {
    MyTheme {
        Scaffold { contentPadding ->
            Box(
                Modifier
                    .padding(contentPadding)
            ) {
                var text by rememberSaveable { mutableStateOf(searchQuery) }
                val expanded = true
                SearchBar(
                    modifier = Modifier
                        .align(TopCenter)
                        .semantics { traversalIndex = 0f },
                    inputField = { /*...*/ }
                }
                BackHandler {
                    onViewEvent(OnBackPress)
                }
            }
        }
    }
}

... and then pass it through the view model and in the fragment pop it from the backstack and finish the hosting activity:

SearchFragment.kt:

class SearchFragment : Fragment() {

    companion object {
        const val FRAGMENT_TAG = "SEARCH_FRAGMENT_TAG"

        fun replaceAtBackStack(fragmentManager: FragmentManager, @IdRes containerViewId: Int) {
            fragmentManager.commit {
                fragmentManager.popBackStack(FRAGMENT_TAG, FragmentManager.POP_BACK_STACK_INCLUSIVE)
                fragmentManager.replaceFragment(containerViewId, SearchFragment(), FRAGMENT_TAG, FRAGMENT_TAG)
            }
        }
    }

    private val viewModelFactory by lazy { SearchViewModelFactory(/*...*/) }
    private val viewModel: SearchViewModel by viewModels { viewModelFactory }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View = 
      inflater.inflate(R.layout.fragment_search, container, false).apply {
        findViewById<ComposeView>(R.id.search_view).apply {
            setViewCompositionStrategy(DisposeOnViewTreeLifecycleDestroyed)
            setContent {
                SearchScreen(
                    searchQuery = viewModel.searchQuery,
                    state = viewModel.searchResultsState.collectAsState().value,
                    onViewEvent = viewModel::onViewEvent,
                )
            }
            isClickable = true
        }
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        observeViewModel()
    }

    private fun observeViewModel() {
        viewModel.navigateBack.observe(viewLifecycleOwner) {
            parentFragmentManager.popBackStack(FRAGMENT_TAG, FragmentManager.POP_BACK_STACK_INCLUSIVE)
            requireActivity().finish()
        }
    }
}