Implement full predictive back support and replace tabs with HorizontalPager

app/shared/src/commonMain/kotlin/ru/fromchat/ui/AboutScreen.kt

@@ -47,6 +47,10 @@ import ru.fromchat.app_name
 import ru.fromchat.back
 import ru.fromchat.ui.branding.FromChatBrandTitle
 
+/**
+ * About is a root [androidx.navigation.NavHost] destination; system and predictive back (see
+ * manifest `enableOnBackInvokedCallback`) pop via the same stack as [LocalNavController.navigateUp].
+ */
 private const val URL_TELEGRAM = "https://t.me/fromchat_ch"
 private const val URL_MAX = "https://maxgate.io/fromchat_ch"
 private const val URL_WEBSITE = "https://fromchat.ru"

app/shared/src/commonMain/kotlin/ru/fromchat/ui/App.kt

@@ -4,6 +4,8 @@ import androidx.compose.animation.AnimatedContentTransitionScope.SlideDirection.
 import androidx.compose.animation.AnimatedContentTransitionScope.SlideDirection.Companion.Start
 import androidx.compose.animation.SharedTransitionLayout
 import androidx.compose.animation.core.tween
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
 import coil3.ImageLoader
 import coil3.compose.setSingletonImageLoaderFactory
 import coil3.svg.SvgDecoder
@@ -38,7 +40,9 @@ import ru.fromchat.ui.auth.LoginScreen
 import ru.fromchat.ui.auth.RegisterScreen
 import ru.fromchat.ui.chat.PublicChatScreen
 import ru.fromchat.ui.debug.DebugApiScreen
-import ru.fromchat.ui.dm.DmContainerScreen
+import ru.fromchat.ui.dm.DmChatRoute
+import ru.fromchat.ui.dm.DmNav
+import ru.fromchat.ui.dm.DmProfileRoute
 import ru.fromchat.ui.main.MainScreen
 import ru.fromchat.ui.profile.ProfileScreen
 import ru.fromchat.ui.setup.ServerConfigScreen
@@ -237,7 +241,7 @@ fun App(scrollToMessageId: Int? = null, startAtPublicChat: Boolean = false) {
                         ProfileScreen(
                             userId = userId,
                             onBack = { navController.navigateUp() },
-                            onChat = { navController.navigate("dm/$it") },
+                            onChat = { navController.navigate(DmNav.chatRoute(it)) },
                             sharedTransitionScope = this@SharedTransitionLayout,
                             animatedVisibilityScope = this@composable,
                             useSharedElementFromNavigation = useSharedElement,
@@ -245,11 +249,61 @@ fun App(scrollToMessageId: Int? = null, startAtPublicChat: Boolean = false) {
                         )
                     }
 
-                    composable("dm/{otherUserId}") { backStackEntry ->
+                    val dmChatProfileFade = tween<Float>(durationMillis = 280)
-                        val otherUserId = backStackEntry.savedStateHandle.get<String>("otherUserId")?.toIntOrNull() ?: 0
+
-                        DmContainerScreen(
+                    composable(
+                        route = DmNav.CHAT_ROUTE,
+                        arguments = listOf(navArgument("otherUserId") { type = NavType.StringType }),
+                        enterTransition = {
+                            when (initialState.destination.route) {
+                                DmNav.PROFILE_ROUTE -> fadeIn(animationSpec = dmChatProfileFade)
+                                else -> slideIntoContainer(Start, animationSpec = animationSpec)
+                            }
+                        },
+                        exitTransition = {
+                            when (targetState.destination.route) {
+                                DmNav.PROFILE_ROUTE -> fadeOut(animationSpec = dmChatProfileFade)
+                                else -> slideOutOfContainer(Start, animationSpec = animationSpec)
+                            }
+                        },
+                        popEnterTransition = {
+                            when (initialState.destination.route) {
+                                DmNav.PROFILE_ROUTE -> fadeIn(animationSpec = dmChatProfileFade)
+                                else -> slideIntoContainer(End, animationSpec = animationSpec)
+                            }
+                        },
+                        popExitTransition = {
+                            when (targetState.destination.route) {
+                                DmNav.PROFILE_ROUTE -> fadeOut(animationSpec = dmChatProfileFade)
+                                else -> slideOutOfContainer(End, animationSpec = animationSpec)
+                            }
+                        },
+                    ) { entry ->
+                        val otherUserId = entry.savedStateHandle.get<String>("otherUserId")?.toIntOrNull() ?: 0
+                        if (otherUserId <= 0) return@composable
+                        DmChatRoute(
                             otherUserId = otherUserId,
-                            onBack = { navController.navigateUp() }
+                            navController = navController,
+                            sharedTransitionScope = this@SharedTransitionLayout,
+                            animatedVisibilityScope = this,
+                        )
+                    }
+
+                    composable(
+                        route = DmNav.PROFILE_ROUTE,
+                        arguments = listOf(navArgument("otherUserId") { type = NavType.StringType }),
+                        enterTransition = { fadeIn(animationSpec = dmChatProfileFade) },
+                        exitTransition = { fadeOut(animationSpec = dmChatProfileFade) },
+                        popEnterTransition = { fadeIn(animationSpec = dmChatProfileFade) },
+                        popExitTransition = { fadeOut(animationSpec = dmChatProfileFade) },
+                    ) { entry ->
+                        val otherUserId = entry.savedStateHandle.get<String>("otherUserId")?.toIntOrNull() ?: 0
+                        if (otherUserId <= 0) return@composable
+                        DmProfileRoute(
+                            otherUserId = otherUserId,
+                            navController = navController,
+                            sharedTransitionScope = this@SharedTransitionLayout,
+                            animatedVisibilityScope = this,
                         )
                     }
 

app/shared/src/commonMain/kotlin/ru/fromchat/ui/debug/DebugApiScreenContent.kt

@@ -37,6 +37,7 @@ import ru.fromchat.crypto.IdentityKeyManager
 import ru.fromchat.crypto.decryptEnvelope
 import ru.fromchat.crypto.transport.TransportCrypto
 import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.dm.DmNav
 
 /**
  * English-only copy: the Debug API screen is not for end users; strings are not in compose
@@ -170,7 +171,7 @@ fun DebugApiScreenContent() {
                 modifier = Modifier.fillMaxWidth()
             )
             Button(
-                onClick = { navController.navigate("dm/2") },
+                onClick = { navController.navigate(DmNav.chatRoute(2)) },
                 modifier = Modifier.fillMaxWidth()
             ) {
                 Text(text = DebugScreenText.OpenAction)

app/shared/src/commonMain/kotlin/ru/fromchat/ui/dm/DmContainerScreen.kt

@@ -1,96 +0,0 @@
-package ru.fromchat.ui.dm
-
-import androidx.compose.animation.AnimatedContent
-import androidx.compose.animation.SharedTransitionLayout
-import androidx.compose.animation.core.tween
-import androidx.compose.animation.fadeIn
-import androidx.compose.animation.fadeOut
-import androidx.compose.animation.togetherWith
-import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.runtime.Composable
-import androidx.compose.runtime.LaunchedEffect
-import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableStateOf
-import androidx.compose.runtime.remember
-import androidx.compose.runtime.rememberCoroutineScope
-import androidx.compose.runtime.setValue
-import androidx.compose.ui.Modifier
-import ru.fromchat.ui.BackHandler
-import ru.fromchat.ui.HapticFeedbackEvent
-import ru.fromchat.ui.rememberHapticFeedback
-import ru.fromchat.ui.profile.ProfileScreen
-
-private const val TRANSITION_MS = 320
-
-@Composable
-fun DmContainerScreen(
-    otherUserId: Int,
-    onBack: () -> Unit,
-    modifier: Modifier = Modifier
-) {
-    var showProfile by remember { mutableStateOf(false) }
-    val scope = rememberCoroutineScope()
-    val haptic = rememberHapticFeedback()
-    val panel = remember(otherUserId) {
-        DmPanelCache.getOrCreate(otherUserId, scope)
-    }
-    var panelState by remember(panel) { mutableStateOf(panel.getState()) }
-
-    LaunchedEffect(panel) {
-        panel.setOnStateChange { panelState = it }
-        panelState = panel.getState()
-    }
-
-    val initialDisplayName = panelState.titleAvatar?.displayName?.takeIf { it.isNotBlank() }
-        ?: panelState.title.takeIf { it.isNotBlank() }
-
-    BackHandler(enabled = showProfile) {
-        haptic(HapticFeedbackEvent.ProfileClosed)
-        showProfile = false
-    }
-
-    // One stable key per DM to match the same avatar in chat + profile.
-    val sharedAvatarKey = remember(otherUserId) { "dm-avatar-$otherUserId" }
-
-    SharedTransitionLayout(modifier = modifier.fillMaxSize()) {
-        AnimatedContent(
-            targetState = showProfile,
-            transitionSpec = {
-                fadeIn(animationSpec = tween(TRANSITION_MS)) togetherWith
-                    fadeOut(animationSpec = tween(TRANSITION_MS))
-            },
-            label = "dm_profile"
-        ) { targetState ->
-            if (!targetState) {
-                DmScreen(
-                    panel = panel,
-                    modifier = Modifier.fillMaxSize(),
-                    onTitleClick = {
-                        haptic(HapticFeedbackEvent.ProfileOpened)
-                        showProfile = true
-                    },
-                    sharedTransitionScope = this@SharedTransitionLayout,
-                    animatedVisibilityScope = this@AnimatedContent,
-                    sharedAvatarKey = sharedAvatarKey
-                )
-            } else {
-                ProfileScreen(
-                    userId = otherUserId,
-                    onBack = {
-                        haptic(HapticFeedbackEvent.ProfileClosed)
-                        showProfile = false
-                    },
-                    onChat = { _ ->
-                        haptic(HapticFeedbackEvent.ProfileClosed)
-                        showProfile = false
-                    },
-                    modifier = Modifier.fillMaxSize(),
-                    sharedTransitionScope = this@SharedTransitionLayout,
-                    animatedVisibilityScope = this@AnimatedContent,
-                    sharedAvatarKey = sharedAvatarKey,
-                    initialDisplayName = initialDisplayName
-                )
-            }
-        }
-    }
-}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/dm/DmNav.kt

@@ -0,0 +1,85 @@
+package ru.fromchat.ui.dm
+
+import androidx.compose.animation.AnimatedVisibilityScope
+import androidx.compose.animation.SharedTransitionScope
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.ui.Modifier
+import androidx.navigation.NavController
+import ru.fromchat.ui.HapticFeedbackEvent
+import ru.fromchat.ui.profile.ProfileScreen
+import ru.fromchat.ui.rememberHapticFeedback
+
+/** Route patterns and builders for DM chat + in-DM profile (stacked for predictive / system back). */
+object DmNav {
+    const val CHAT_ROUTE = "dm/{otherUserId}/chat"
+    const val PROFILE_ROUTE = "dm/{otherUserId}/profile"
+
+    fun chatRoute(otherUserId: Int) = "dm/$otherUserId/chat"
+
+    fun profileRoute(otherUserId: Int) = "dm/$otherUserId/profile"
+}
+
+private const val DM_AVATAR_KEY_PREFIX = "dm-avatar-"
+
+@Composable
+fun DmChatRoute(
+    otherUserId: Int,
+    navController: NavController,
+    sharedTransitionScope: SharedTransitionScope,
+    animatedVisibilityScope: AnimatedVisibilityScope,
+    modifier: Modifier = Modifier,
+) {
+    val scope = rememberCoroutineScope()
+    val panel = remember(otherUserId, scope) { DmPanelCache.getOrCreate(otherUserId, scope) }
+    val haptic = rememberHapticFeedback()
+    val sharedAvatarKey = remember(otherUserId) { "$DM_AVATAR_KEY_PREFIX$otherUserId" }
+
+    DmScreen(
+        panel = panel,
+        modifier = modifier.fillMaxSize(),
+        onTitleClick = {
+            haptic(HapticFeedbackEvent.ProfileOpened)
+            navController.navigate(DmNav.profileRoute(otherUserId))
+        },
+        sharedTransitionScope = sharedTransitionScope,
+        animatedVisibilityScope = animatedVisibilityScope,
+        sharedAvatarKey = sharedAvatarKey
+    )
+}
+
+@Composable
+fun DmProfileRoute(
+    otherUserId: Int,
+    navController: NavController,
+    sharedTransitionScope: SharedTransitionScope,
+    animatedVisibilityScope: AnimatedVisibilityScope,
+    modifier: Modifier = Modifier,
+) {
+    val scope = rememberCoroutineScope()
+    val panel = remember(otherUserId, scope) { DmPanelCache.getOrCreate(otherUserId, scope) }
+    val haptic = rememberHapticFeedback()
+    val sharedAvatarKey = remember(otherUserId) { "$DM_AVATAR_KEY_PREFIX$otherUserId" }
+    val stateSnapshot = panel.getState()
+    val initialDisplayName = stateSnapshot.titleAvatar?.displayName?.takeIf { it.isNotBlank() }
+        ?: stateSnapshot.title.takeIf { it.isNotBlank() }
+
+    ProfileScreen(
+        userId = otherUserId,
+        onBack = {
+            haptic(HapticFeedbackEvent.ProfileClosed)
+            navController.popBackStack()
+        },
+        onChat = {
+            haptic(HapticFeedbackEvent.ProfileClosed)
+            navController.popBackStack()
+        },
+        modifier = modifier.fillMaxSize(),
+        sharedTransitionScope = sharedTransitionScope,
+        animatedVisibilityScope = animatedVisibilityScope,
+        sharedAvatarKey = sharedAvatarKey,
+        initialDisplayName = initialDisplayName
+    )
+}

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/ChatsTab.kt

@@ -51,6 +51,7 @@ import ru.fromchat.api.db.MessageCacheStore
 import ru.fromchat.net.NetworkConnectivity
 import ru.fromchat.ui.ConnectingEllipsis
 import ru.fromchat.ui.LocalNavController
+import ru.fromchat.ui.dm.DmNav
 import ru.fromchat.ui.branding.FromChatBrandTitle
 import ru.fromchat.ui.chat.Avatar
 
@@ -287,7 +288,7 @@ fun ChatsTab() {
                     },
                     modifier = Modifier.clickable {
                         if (conv.otherUserId != 0) {
-                            navController.navigate("dm/${conv.otherUserId}")
+                            navController.navigate(DmNav.chatRoute(conv.otherUserId))
                         }
                     }
                 )

app/shared/src/commonMain/kotlin/ru/fromchat/ui/main/MainScreen.kt

@@ -1,12 +1,13 @@
 package ru.fromchat.ui.main
 
-import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.WindowInsets
 import androidx.compose.foundation.layout.WindowInsetsSides
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.imePadding
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.safeDrawing
+import androidx.compose.foundation.pager.HorizontalPager
+import androidx.compose.foundation.pager.rememberPagerState
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.Chat
 import androidx.compose.material.icons.filled.Contacts
@@ -18,11 +19,9 @@ import androidx.compose.material3.NavigationBarItem
 import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
-import androidx.compose.runtime.getValue
+import androidx.compose.runtime.rememberCoroutineScope
-import androidx.compose.runtime.mutableStateOf
-import androidx.compose.runtime.saveable.rememberSaveable
-import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import kotlinx.coroutines.launch
 import org.jetbrains.compose.resources.stringResource
 import ru.fromchat.Res
 import ru.fromchat.*
@@ -30,35 +29,56 @@ import ru.fromchat.api.ApiClient
 import ru.fromchat.utils.exclude
 import ru.fromchat.ui.profile.ProfileScreen
 
+private const val PAGE_CHATS = 0
+private const val PAGE_CONTACTS = 1
+private const val PAGE_SETTINGS = 2
+private const val PAGE_PROFILE = 3
+private const val PAGE_COUNT = 4
+
 @Suppress("AssignedValueIsNeverRead")
 @Composable
 fun MainScreen(onLogout: () -> Unit = {}) {
-    var selectedTab by rememberSaveable { mutableStateOf("chats") }
+    val pagerState = rememberPagerState(
+        initialPage = PAGE_CHATS,
+        pageCount = { PAGE_COUNT },
+    )
+    val scope = rememberCoroutineScope()
+
+    // Pager is the single source of truth; tabs only call animateScrollToPage (no write/read loop).
+    val selectedPage = pagerState.currentPage
 
     Scaffold(
         bottomBar = {
             NavigationBar {
                 NavigationBarItem(
-                    selected = selectedTab == "chats",
+                    selected = selectedPage == PAGE_CHATS,
-                    onClick = { selectedTab = "chats" },
+                    onClick = {
+                        scope.launch { pagerState.animateScrollToPage(PAGE_CHATS) }
+                    },
                     label = { Text(stringResource(Res.string.chats)) },
                     icon = { Icon(Icons.AutoMirrored.Filled.Chat, contentDescription = null) }
                 )
                 NavigationBarItem(
-                    selected = selectedTab == "contacts",
+                    selected = selectedPage == PAGE_CONTACTS,
-                    onClick = { selectedTab = "contacts" },
+                    onClick = {
+                        scope.launch { pagerState.animateScrollToPage(PAGE_CONTACTS) }
+                    },
                     label = { Text(stringResource(Res.string.contacts)) },
                     icon = { Icon(Icons.Filled.Contacts, contentDescription = null) }
                 )
                 NavigationBarItem(
-                    selected = selectedTab == "settings",
+                    selected = selectedPage == PAGE_SETTINGS,
-                    onClick = { selectedTab = "settings" },
+                    onClick = {
+                        scope.launch { pagerState.animateScrollToPage(PAGE_SETTINGS) }
+                    },
                     label = { Text(stringResource(Res.string.settings)) },
                     icon = { Icon(Icons.Filled.Settings, contentDescription = null) }
                 )
                 NavigationBarItem(
-                    selected = selectedTab == "profile",
+                    selected = selectedPage == PAGE_PROFILE,
-                    onClick = { selectedTab = "profile" },
+                    onClick = {
+                        scope.launch { pagerState.animateScrollToPage(PAGE_PROFILE) }
+                    },
                     label = { Text(stringResource(Res.string.profile)) },
                     icon = { Icon(Icons.Filled.Person, contentDescription = null) }
                 )
@@ -67,29 +87,30 @@ fun MainScreen(onLogout: () -> Unit = {}) {
         contentWindowInsets = WindowInsets.safeDrawing.exclude(WindowInsetsSides.Top),
         modifier = Modifier.imePadding()
     ) { innerPadding ->
-        Column(
+        HorizontalPager(
-            Modifier
+            state = pagerState,
+            modifier = Modifier
                 .fillMaxSize()
-                .padding(innerPadding)
+                .padding(innerPadding),
-        ) {
+            beyondViewportPageCount = 1,
-            when (selectedTab) {
+        ) { page ->
-                "chats" -> ChatsTab()
+            when (page) {
-                "contacts" -> {
+                PAGE_CHATS -> ChatsTab()
-                    Text(stringResource(Res.string.coming_soon))
+                PAGE_CONTACTS -> Text(stringResource(Res.string.coming_soon))
-                }
+                PAGE_SETTINGS -> SettingsTab(onLogout = onLogout)
-                "settings" -> {
+                PAGE_PROFILE -> {
-                    SettingsTab(onLogout = onLogout)
-                }
-                "profile" -> {
                     val currentUserId = ApiClient.user?.id
                     ProfileScreen(
                         userId = currentUserId,
                         onBack = {},
                         onChat = { _ -> },
                         modifier = Modifier.fillMaxSize(),
-                        onOpenSettings = { selectedTab = "settings" }
+                        onOpenSettings = {
+                            scope.launch { pagerState.animateScrollToPage(PAGE_SETTINGS) }
+                        }
                     )
                 }
+                else -> Unit
             }
         }
     }