feat: persist scroll position per channel with Jump to Present button

- Save the top-visible message ID on channel leave, restore via
  scrollIntoView on return (immune to lazy-loaded image reflow)
- Add floating glass-bubble "Jump to Present" button when scrolled
  5000px+ from bottom
- Clear stale scroll anchors when user returns to bottom
- Evict scroll positions alongside channel cache eviction

packages/web/src/components/chat/MessageList.tsx

@@ -49,6 +49,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
   const isLoading = useChatStore((s) => s.isLoading);
   const hasMore = useChatStore((s) => s.hasMore.get(channelId) ?? true);
   const ackChannel = useChatStore((s) => s.ackChannel);
+  const saveScrollPosition = useChatStore((s) => s.saveScrollPosition);
   const bottomRef = useRef<HTMLDivElement>(null);
   const containerRef = useRef<HTMLDivElement>(null);
   const contentRef = useRef<HTMLDivElement>(null);
@@ -56,6 +57,8 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
   const isNearBottomRef = useRef(true);
   const [isLoadingMore, setIsLoadingMore] = useState(false);
   const prevMessagesLength = useRef(0);
+  const prevChannelIdRef = useRef<string>(channelId);
+  const visibleMsgIdRef = useRef<string | null>(null);
   const ackTimerRef = useRef<ReturnType<typeof setTimeout>>();
 
   // Permission check: DM channels always allow history; space channels check READ_MESSAGE_HISTORY
@@ -81,14 +84,39 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
     return () => clearTimeout(ackTimerRef.current);
   }, [channelId, messages.length, lastMessageId, isNearBottom, ackChannel]);
 
-  // Reset scroll tracking on channel switch so initial-load scroll fires
+  // Save scroll anchor (tracked by handleScroll) when leaving a channel, then reset tracking
   useEffect(() => {
-    prevMessagesLength.current = 0;
+    const prevId = prevChannelIdRef.current;
-    setIsNearBottom(true);
+    prevChannelIdRef.current = channelId;
-    isNearBottomRef.current = true;
-  }, [channelId]);
 
-  // Handle scrolling: initial load snaps to bottom, new messages smooth-scroll if near bottom
+    // Save or clear the old channel's scroll position
+    if (prevId && prevId !== channelId) {
+      if (visibleMsgIdRef.current) {
+        // User was scrolled up — save the anchor message
+        saveScrollPosition(prevId, visibleMsgIdRef.current);
+        visibleMsgIdRef.current = null;
+      } else {
+        // User was at bottom — clear any stale saved position so we snap to bottom next time
+        const pos = useChatStore.getState().scrollPositions;
+        if (pos.has(prevId)) {
+          const next = new Map(pos);
+          next.delete(prevId);
+          useChatStore.setState({ scrollPositions: next });
+        }
+      }
+    }
+
+    prevMessagesLength.current = 0;
+
+    // If we have a saved position for the incoming channel, don't mark as near-bottom
+    // — this prevents the ResizeObserver from snapping to bottom before the restore rAF fires
+    const willRestore = useChatStore.getState().scrollPositions.has(channelId);
+    setIsNearBottom(!willRestore);
+    isNearBottomRef.current = !willRestore;
+  }, [channelId, saveScrollPosition]);
+
+  // Handle scrolling: initial load restores position or snaps to bottom,
+  // new messages smooth-scroll if near bottom
   useEffect(() => {
     const prev = prevMessagesLength.current;
     prevMessagesLength.current = messages.length;
@@ -96,18 +124,30 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
     if (messages.length === 0) return;
 
     if (prev === 0) {
-      // Initial load / channel switch — snap to bottom
+      // Initial load / channel switch — restore to saved message anchor or snap to bottom
+      const savedMsgId = useChatStore.getState().scrollPositions.get(channelId);
       requestAnimationFrame(() => {
         const container = containerRef.current;
-        if (container) {
+        if (!container) return;
-          container.scrollTop = container.scrollHeight;
+        if (savedMsgId) {
+          const el = document.getElementById(`msg-${savedMsgId}`);
+          if (el) {
+            el.scrollIntoView({ block: 'start' });
+            const dist = container.scrollHeight - container.scrollTop - container.clientHeight;
+            const near = dist < 5000;
+            setIsNearBottom(near);
+            isNearBottomRef.current = near;
+            return;
           }
+        }
+        // No saved anchor or message not in cache — snap to bottom
+        container.scrollTop = container.scrollHeight;
       });
     } else if (messages.length > prev && isNearBottom) {
       // New messages arrived while near bottom — smooth scroll
       bottomRef.current?.scrollIntoView({ behavior: 'smooth' });
     }
-  }, [messages.length, isNearBottom]);
+  }, [messages.length, isNearBottom, channelId]);
 
   // Auto-scroll when content height grows (embeds/images loading) while near bottom
   const hasMessages = messages.length > 0;
@@ -178,10 +218,24 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
 
     // Check if near bottom
     const distanceFromBottom = container.scrollHeight - container.scrollTop - container.clientHeight;
-    const nearBottom = distanceFromBottom < 100;
+    const nearBottom = distanceFromBottom < 5000;
     setIsNearBottom(nearBottom);
     isNearBottomRef.current = nearBottom;
 
+    // Track top-visible message for scroll position persistence
+    if (!nearBottom) {
+      const containerTop = container.getBoundingClientRect().top;
+      const msgEls = container.querySelectorAll('[id^="msg-"]');
+      for (const el of msgEls) {
+        if (el.getBoundingClientRect().bottom > containerTop) {
+          visibleMsgIdRef.current = el.id.replace('msg-', '');
+          break;
+        }
+      }
+    } else {
+      visibleMsgIdRef.current = null;
+    }
+
     // Load more when scrolled to top
     if (container.scrollTop < 50 && hasMore && !isLoadingMore) {
       setIsLoadingMore(true);
@@ -214,9 +268,10 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
   }
 
   return (
+    <div className="flex-1 relative min-h-0">
       <div
         ref={containerRef}
-      className="flex-1 overflow-y-auto overflow-x-hidden no-scrollbar"
+        className="h-full overflow-y-auto overflow-x-hidden no-scrollbar"
         onScroll={handleScroll}
       >
         {isLoadingMore && (
@@ -256,6 +311,19 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
 
         <div ref={bottomRef} />
       </div>
+
+      {!isNearBottom && messages.length > 0 && (
+        <button
+          onClick={() => bottomRef.current?.scrollIntoView({ behavior: 'smooth' })}
+          className="absolute bottom-20 left-1/2 -translate-x-1/2 z-[120] glass-bubble px-4 py-2 flex items-center gap-2 rounded-full text-txt-secondary hover:text-txt-primary transition-all animate-fade-in cursor-pointer"
+        >
+          <svg width="18" height="18" viewBox="0 0 24 24" fill="currentColor">
+            <path d="M7.41 8.59L12 13.17l4.59-4.58L18 10l-6 6-6-6z" />
+          </svg>
+          <span className="text-[13px] font-medium">Jump to Present</span>
+        </button>
+      )}
+    </div>
   );
 }
 

packages/web/src/stores/chatStore.ts

@@ -32,7 +32,9 @@ interface ChatState {
   unreadChannels: Set<string>;
   realtimeMessageEvents: RealtimeMessageEvent[];
   channelAccessTimes: Map<string, number>;
+  scrollPositions: Map<string, string>;
   setCurrentChannel: (channelId: string | null) => void;
+  saveScrollPosition: (channelId: string, messageId: string) => void;
   setReplyTo: (message: MessageWithUser | null) => void;
   loadMessages: (channelId: string, force?: boolean) => Promise<void>;
   clearAllMessages: () => void;
@@ -83,6 +85,15 @@ export const useChatStore = create<ChatState>((set, get) => ({
   unreadChannels: new Set(),
   realtimeMessageEvents: [],
   channelAccessTimes: new Map(),
+  scrollPositions: new Map(),
+
+  saveScrollPosition: (channelId, messageId) => {
+    set((state) => {
+      const newPositions = new Map(state.scrollPositions);
+      newPositions.set(channelId, messageId);
+      return { scrollPositions: newPositions };
+    });
+  },
 
   setCurrentChannel: (channelId) => {
     set((state) => {
@@ -94,6 +105,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
       // Evict stale channels if we have too many cached
       let newMessages = state.messages;
       let newHasMore = state.hasMore;
+      let newScrollPositions = state.scrollPositions;
       if (state.messages.size > MAX_CACHED_CHANNELS) {
         const entries = [...newAccessTimes.entries()]
           .filter(([id]) => id !== channelId)
@@ -103,10 +115,12 @@ export const useChatStore = create<ChatState>((set, get) => ({
         if (evictIds.size > 0) {
           newMessages = new Map(state.messages);
           newHasMore = new Map(state.hasMore);
+          newScrollPositions = new Map(state.scrollPositions);
           for (const id of evictIds) {
             newMessages.delete(id);
             newHasMore.delete(id);
             newAccessTimes.delete(id);
+            newScrollPositions.delete(id);
           }
         }
       }
@@ -116,6 +130,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
         channelAccessTimes: newAccessTimes,
         messages: newMessages,
         hasMore: newHasMore,
+        scrollPositions: newScrollPositions,
       };
     });
   },
@@ -129,6 +144,7 @@ export const useChatStore = create<ChatState>((set, get) => ({
     unreadChannels: new Set(),
     realtimeMessageEvents: [],
     channelAccessTimes: new Map(),
+    scrollPositions: new Map(),
     currentChannelId: null,
     replyTo: null,
   }),