feat(bots): smart streaming auto-scroll and scroll-to-bottom FAB in group chat (#1013)

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/bots/group/GroupChatScreen.kt

@@ -77,6 +77,9 @@ import androidx.lifecycle.viewmodel.compose.viewModel
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.theme.LocalChatFontScale
 import com.m57.hermescontrol.ui.chat.MarkdownText
+import com.m57.hermescontrol.ui.chat.components.ChatScrollToBottomFab
+import com.m57.hermescontrol.ui.chat.components.rememberChatScrollController
+import com.m57.hermescontrol.ui.chat.components.tailContentKey
 import com.m57.hermescontrol.ui.chat.formatTimestamp
 import com.m57.hermescontrol.ui.common.BotAvatar
 import com.m57.hermescontrol.ui.common.EmptyState
@@ -95,6 +98,8 @@ fun GroupChatScreen(
 ) {
     val state by viewModel.uiState.collectAsState()
     val listState = rememberLazyListState()
+    val scrollScope = rememberCoroutineScope()
+    val scrollController = rememberChatScrollController(listState, scrollScope)
     var inputFieldValue by rememberSaveable(stateSaver = TextFieldValue.Saver) {
         mutableStateOf(TextFieldValue(""))
     }
@@ -104,6 +109,7 @@ fun GroupChatScreen(
     val handleSend = {
         val text = inputFieldValue.text
         if (text.isNotBlank()) {
+            scrollController.jumpToBottom()
             viewModel.sendMessage(text)
             inputFieldValue = TextFieldValue("")
         }
@@ -113,10 +119,32 @@ fun GroupChatScreen(
         viewModel.setGroup(groupName)
     }
 
-    LaunchedEffect(state.messages.size) {
+    LaunchedEffect(Unit) {
-        if (state.messages.isNotEmpty()) {
+        scrollController.observeUserScrollPosition()
-            listState.animateScrollToItem(state.messages.size - 1)
     }
+
+    val streamingMessage = state.messages.find { it.isStreaming }
+    val tailKey =
+        remember(
+            state.messages.size,
+            streamingMessage?.id,
+            streamingMessage?.text?.length,
+            state.activeSpeaker,
+        ) {
+            tailContentKey(
+                messages = state.messages,
+                streamingMessage = streamingMessage?.text,
+                isThinking = state.activeSpeaker != null,
+                subagentIndicators = emptyList<Any>(),
+                clarifyRequest = null,
+            )
+        }
+
+    LaunchedEffect(tailKey) {
+        scrollController.onTailChanged(
+            tailKey = tailKey,
+            messageCount = state.messages.size,
+        )
     }
 
     HermesScaffold(
@@ -204,13 +232,16 @@ fun GroupChatScreen(
                             )
                         }
 
-                    CompositionLocalProvider(LocalDensity provides chatDensity) {
+                    Box(
-                        LazyColumn(
-                            state = listState,
                         modifier =
                             Modifier
                                 .weight(1f)
                                 .fillMaxWidth(),
+                    ) {
+                        CompositionLocalProvider(LocalDensity provides chatDensity) {
+                            LazyColumn(
+                                state = listState,
+                                modifier = Modifier.fillMaxSize(),
                                 contentPadding = PaddingValues(horizontal = 16.dp, vertical = 8.dp),
                                 verticalArrangement = Arrangement.spacedBy(12.dp),
                             ) {
@@ -242,6 +273,13 @@ fun GroupChatScreen(
                                 }
                             }
                         }
+
+                        ChatScrollToBottomFab(
+                            show = !scrollController.isFollowingBottom && state.messages.isNotEmpty(),
+                            pendingCount = scrollController.pendingCount,
+                            onScrollToBottom = { scrollController.resumeFollowing() },
+                        )
+                    }
                 }
             }