Fix scrollback snapping back to bottom after scrolling up

Wheel-up now clears stickToBottom immediately on the raw scroll event, wheel-down re-checks the resting position, and the auto-scroll effect re-checks stickToBottom right before scrollToItem(0). The scroll-state effect could miss a wheel scroll that began and ended within one frame, so the next incoming line pulled the buffer back to the bottom.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JW57ZvxRmxcDF7AFACLSoV

src/main/kotlin/ui/ChannelScreen.kt

@@ -818,7 +818,7 @@ fun MessageScrollback(
                 .first { item ->
                     item == null || (item.index < capturedList.size && item.key == capturedList[item.index].seq)
                 }
-            listState.scrollToItem(0)
+            if (stickToBottom) listState.scrollToItem(0)
         }
     }
 
@@ -923,6 +923,31 @@ fun MessageScrollback(
             .height(snappedHeight)
             .onGloballyPositioned { containerCoords[0] = it }
             .pointerHoverIcon(PointerIcon.Text)
+            // Wheel-up means the user is reading backscroll: stop sticking to the bottom RIGHT NOW,
+            // on the raw wheel event. The scroll-state effect up top can miss a wheel scroll that starts
+            // and finishes inside one frame (isScrollInProgress is never seen as true), which left
+            // stickToBottom set and let the next incoming line yank the buffer back down.
+            // Wheel-down re-checks the resting position shortly after and re-enables sticking if the
+            // user is back at the bottom.
+            .pointerInput(Unit) {
+                awaitPointerEventScope {
+                    while (true) {
+                        val event = awaitPointerEvent(PointerEventPass.Initial)
+                        if (event.type == PointerEventType.Scroll) {
+                            val dy = event.changes.firstOrNull()?.scrollDelta?.y ?: 0f
+                            if (dy < 0f) {
+                                stickToBottom = false
+                            } else if (dy > 0f) {
+                                coroutineScope.launch {
+                                    delay(200)
+                                    stickToBottom = listState.firstVisibleItemIndex == 0 &&
+                                        listState.firstVisibleItemScrollOffset <= stickToBottomThresholdPx
+                                }
+                            }
+                        }
+                    }
+                }
+            }
             // Remembers the row under a right-click for the menu's "Copy line". Watches the
             // Initial pass, because ContextMenuArea takes the right-press there, before
             // the handler below would see it.