Back-to-bottom button: chevron FAB appears when scrolled up, tap glides to latest; new messages no longer yank the view while reading history; lane switch always lands on latest

app/shared/src/commonMain/kotlin/iris/ui/screens/ChatScreen.kt

@@ -41,12 +41,15 @@ import androidx.compose.foundation.text.BasicTextField
 import androidx.compose.foundation.text.KeyboardActions
 import androidx.compose.foundation.text.KeyboardOptions
 import androidx.compose.foundation.verticalScroll
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.KeyboardArrowDown
 import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.Button
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.DrawerValue
 import androidx.compose.material3.DropdownMenu
 import androidx.compose.material3.DropdownMenuItem
+import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.ModalDrawerSheet
@@ -64,9 +67,11 @@ import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.snapshotFlow
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
 import androidx.compose.ui.draw.clip
 import androidx.compose.ui.focus.FocusRequester
 import androidx.compose.ui.focus.focusRequester
@@ -145,6 +150,25 @@ fun ChatScreen(controller: IrisController) {
     val threads = channels.filter { it.kind == "thread" && it.parentChatId == currentChatId }
 
     val listState = rememberLazyListState()
+    // "At the bottom" while the last row's bottom edge is within a small
+    // tolerance of its rest position (16dp above the viewport bottom, from the
+    // list's content padding). Scrolling up by more than that reveals the
+    // back-to-bottom button and stops new messages from auto-scrolling the view.
+    val bottomThresholdPx = 8.dp.value * LocalDensity.current.density
+    var isAtBottom by remember { mutableStateOf(true) }
+    var isAutoScrolling by remember { mutableStateOf(false) }
+    LaunchedEffect(listState) {
+        snapshotFlow {
+            val info = listState.layoutInfo
+            val last = info.visibleItemsInfo.lastOrNull()
+            if (last == null) {
+                true
+            } else {
+                val gap = (info.viewportEndOffset - (last.offset + last.size)).toFloat()
+                last.index == info.totalItemsCount - 1 && gap >= bottomThresholdPx
+            }
+        }.collect { isAtBottom = it }
+    }
     var input by remember { mutableStateOf("") }
     var showSearch by remember { mutableStateOf(false) }
     var showNewChannel by remember { mutableStateOf(false) }
@@ -187,9 +211,26 @@ fun ChatScreen(controller: IrisController) {
         focusManager.clearFocus(force = true)
     }
 
-    LaunchedEffect(items.size, typing) {
+    // (Re)entering a lane always lands on the latest message.
+    LaunchedEffect(currentLane) {
         val last = items.size - 1
-        if (last >= 0) listState.animateScrollToItem(last)
+        if (last >= 0) listState.scrollToItem(last)
+    }
+
+    // Follow the bottom only while the user is already there; if they've
+    // scrolled up to read history, new messages must not yank the view back
+    // (the back-to-bottom button gets them back).
+    LaunchedEffect(items.size, typing) {
+        if (!isAtBottom) return@LaunchedEffect
+        val last = items.size - 1
+        if (last >= 0) {
+            isAutoScrolling = true
+            try {
+                listState.animateScrollToItem(last)
+            } finally {
+                isAutoScrolling = false
+            }
+        }
     }
 
     // M6: keep the rail selection in sync with the current channel.
@@ -379,6 +420,38 @@ fun ChatScreen(controller: IrisController) {
                         }
                     }
                 }
+
+                // Back-to-bottom button: appears once the user has scrolled up
+                // to read history; a tap glides the view back to the latest
+                // message (smooth but quick).
+                if (!isAtBottom && !isAutoScrolling) {
+                    val alpha = remember { Animatable(0f) }
+                    LaunchedEffect(Unit) { alpha.animateTo(1f, tween(150)) }
+                    Box(
+                        modifier = Modifier
+                            .align(Alignment.BottomEnd)
+                            .padding(end = 16.dp, bottom = 12.dp)
+                            .alpha(alpha.value)
+                            .size(40.dp)
+                            .clip(CircleShape)
+                            .background(IrisColors.surface)
+                            .border(1.dp, IrisColors.divider, CircleShape)
+                            .clickable {
+                                val last = listState.layoutInfo.totalItemsCount - 1
+                                if (last >= 0) {
+                                    drawerScope.launch { listState.animateScrollToItem(last) }
+                                }
+                            },
+                        contentAlignment = Alignment.Center,
+                    ) {
+                        Icon(
+                            imageVector = Icons.Filled.KeyboardArrowDown,
+                            contentDescription = "Scroll to latest",
+                            tint = IrisColors.textSecondary,
+                            modifier = Modifier.size(22.dp),
+                        )
+                    }
+                }
             }
 
             // Pending attachments (M4)