AppNavigationScaffold.kt

@OptIn(ExperimentalMaterial3Api::class)
@Composable
fun AppNavigationScaffold() {
    val topBarHeight = 64.dp
    val bottomBarHeight = 80.dp
    val navController = rememberNavController()
    var barsVisible by remember { mutableStateOf(true) }

    val animatedTopOffset by animateDpAsState(
        targetValue = if (barsVisible) 0.dp else -topBarHeight,
        animationSpec = tween(durationMillis = 200),
        label = "TopBarOffset"
    )

    val animatedBottomOffset by animateDpAsState(
        targetValue = if (barsVisible) 0.dp else bottomBarHeight,
        animationSpec = tween(durationMillis = 200),
        label = "BottomBarOffset"
    )

    Scaffold(
        topBar = {
            TopAppBar(
                title = { Text("共通タイトル") },
                modifier = Modifier.height(topBarHeight).offset(y = animatedTopOffset)
            )
        },
        bottomBar = {
            NavigationBar(
                modifier = Modifier.height(bottomBarHeight).offset(y = animatedBottomOffset)
            ) {
                NavigationBarItem(
                    icon = { Icon(Icons.Default.Home, null) },
                    label = { Text("Home") },
                    selected = true,
                    onClick = { navController.navigate("home") }
                )
                NavigationBarItem(
                    icon = { Icon(Icons.Default.Settings, null) },
                    label = { Text("Setting") },
                    selected = false,
                    onClick = { navController.navigate("setting") }
                )
            }
        }
    ) { innerPadding ->
        NavHost(
            navController = navController,
            startDestination = "home",
            modifier = Modifier.fillMaxSize()
        ) {
            composable("home") {
                HomeScreen(
                    topBarHeight = topBarHeight,
                    bottomBarHeight = bottomBarHeight,
                    onScrollDirectionChanged = { visible -> barsVisible = visible }
                )
            }
            composable("setting") {
                SettingScreen(
                    topBarHeight = topBarHeight,
                    bottomBarHeight = bottomBarHeight,
                    onScrollDirectionChanged = { visible -> barsVisible = visible }
                )
            }
        }
    }
}

/**
 * ホーム画面
 */
@Composable
fun HomeScreen(
    topBarHeight: Dp,
    bottomBarHeight: Dp,
    onScrollDirectionChanged: (Boolean) -> Unit
) {
    val scrollState = rememberLazyListState()

    // 拡張関数をここでサクッと呼び出す
    scrollState.trackScrollDirection(onScrollDirectionChanged)

    LazyColumn(
        state = scrollState,
        modifier = Modifier.fillMaxSize(),
        contentPadding = PaddingValues(top = topBarHeight, bottom = bottomBarHeight) 
    ) {
        items(50) { index ->
            ListItem(headlineContent = { Text("ホームコンテンツ $index") })
            HorizontalDivider()
        }
    }
}

/**
 * 設定画面
 */
@Composable
fun SettingScreen(
    topBarHeight: Dp,
    bottomBarHeight: Dp,
    onScrollDirectionChanged: (Boolean) -> Unit
) {
    val scrollState = rememberLazyListState()

    // どの画面でも state から直感的に生やせる
    scrollState.trackScrollDirection(onScrollDirectionChanged)

    LazyColumn(
        state = scrollState,
        modifier = Modifier.fillMaxSize(),
        contentPadding = PaddingValues(top = topBarHeight, bottom = bottomBarHeight) 
    ) {
        items(30) { index ->
            ListItem(headlineContent = { Text("設定項目 $index") })
            HorizontalDivider()
        }
    }
}

ScrollAwareLazyColumn.kt

@Composable
fun ScrollAwareLazyColumn(
    topBarHeight: Dp,
    bottomBarHeight: Dp,
    onScrollDirectionChanged: (Boolean) -> Unit,
    modifier: Modifier = Modifier,
    state: LazyListState = rememberLazyListState(), // 外部から制御したい時のために一応保持
    content: LazyListScope.() -> Unit
) {
    // スクロール方向の検知ロジックをここに内包
    LaunchedEffect(state) {
        var previousPosition = 0
        snapshotFlow { state.firstVisibleItemIndex * 10000 + state.firstVisibleItemScrollOffset }
            .collect { currentPosition ->
                if (state.isScrollInProgress) {
                    val delta = currentPosition - previousPosition
                    if (abs(delta) > 15) {
                        val visible = currentPosition < previousPosition
                        onScrollDirectionChanged(visible)
                    }
                }
                previousPosition = currentPosition
            }
    }

    LazyColumn(
        state = state,
        modifier = modifier.fillMaxSize(),
        // 共通の上下余白をここで強制適用
        contentPadding = PaddingValues(top = topBarHeight, bottom = bottomBarHeight),
        content = content
    )
}

TrackScrollDirection.kt

@Composable
fun LazyListState.trackScrollDirection(onScrollDirectionChanged: (Boolean) -> Unit) {
    LaunchedEffect(this) {
        var previousPosition = 0
        snapshotFlow { firstVisibleItemIndex * 10000 + firstVisibleItemScrollOffset }
            .collect { currentPosition ->
                if (isScrollInProgress) {
                    val delta = currentPosition - previousPosition
                    if (abs(delta) > 15) {
                        val visible = currentPosition < previousPosition
                        onScrollDirectionChanged(visible)
                    }
                }
                previousPosition = currentPosition
            }
    }
}

TrackScrollDirectionModifier.kt

fun Modifier.trackScrollDirection(
    scrollState: LazyListState,
    onScrollDirectionChanged: (Boolean) -> Unit
): Modifier = composed {
    LaunchedEffect(scrollState) {
        var previousPosition = 0
        snapshotFlow { scrollState.firstVisibleItemIndex * 10000 + scrollState.firstVisibleItemScrollOffset }
            .collect { currentPosition ->
                if (scrollState.isScrollInProgress) {
                    val delta = currentPosition - previousPosition
                    if (abs(delta) > 15) {
                        val visible = currentPosition < previousPosition
                        onScrollDirectionChanged(visible)
                    }
                }
                previousPosition = currentPosition
            }
    }
    this
}

Compose State を Flow に変換する唯一の方法、それが snapshotFlow
https://android.benigumo.com/20260706/snapshotflow/

【Jetpack Compose】スクロール位置を正しく復元する
https://android.benigumo.com/20260624/restore-scroll-position/