[CLNP-8108][fix]: Fixed a bug where the message list did not scroll to the startingPoint set on the GroupChannel (#1397)

Fixes [CLNP-8108](https://sendbird.atlassian.net/browse/CLNP-8108)

### Changelogs
- Fixed a bug where the message list did not scroll to the startingPoint
set on the GroupChannel

### Checklist

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creating the PR. If unsure, ask the members.
This is a reminder of what we look for before merging your code.

- [x] **All tests pass locally with my changes**
- [ ] **I have added tests that prove my fix is effective or that my
feature works**
- [ ] **Public components / utils / props are appropriately exported**
- [ ] I have added necessary documentation (if appropriate)


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[CLNP-8108]:
https://sendbird.atlassian.net/browse/CLNP-8108?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: danney-chun

src/modules/GroupChannel/context/GroupChannelProvider.tsx

@@ -302,10 +302,10 @@ const GroupChannelManager :React.FC<React.PropsWithChildren<GroupChannelProvider
 
   // Starting point handling
   useEffect(() => {
-    if (typeof startingPoint === 'number' && messageDataSource.initialized) {
+    if (typeof startingPoint === 'number' && state.initialized) {
       actions.scrollToMessage(startingPoint, 0, false, false);
     }
-  }, [messageDataSource.initialized, startingPoint]);
+  }, [state.initialized, startingPoint]);
 
   // Animated message handling
   useEffect(() => {

src/modules/GroupChannel/context/__tests__/useGroupChannel.spec.tsx

@@ -301,7 +301,12 @@ describe('useGroupChannel', () => {
       });
       it('loads message and scrolls when message does not exist', async () => {
         const mockScrollPubSub = { publish: jest.fn() };
-        const mockResetWithStartingPoint = jest.fn().mockResolvedValue(undefined);
+        const mockResetWithStartingPoint = jest.fn().mockImplementation(async (startingPoint: number) => {
+          mockStore.setState((prev) => ({
+            ...prev,
+            messages: [{ createdAt: startingPoint, messageId: 123 }],
+          } as typeof prev));
+        });
         const mockStore = createMockStore({
           messages: [],
           initialized: true,

src/modules/GroupChannel/context/hooks/useGroupChannel.ts

@@ -126,9 +126,8 @@ export const useGroupChannel = () => {
     clickHandler.deactivate();
 
     setAnimatedMessageId(null);
-    const message = state.messages.find(
-      (it) => it.messageId === messageId || it.createdAt === createdAt,
-    );
+
+    const message = state.messages.find((it) => it.messageId === messageId || it.createdAt === createdAt);
 
     if (message) {
       const topOffset = getMessageTopOffset(message.createdAt);
@@ -137,7 +136,23 @@ export const useGroupChannel = () => {
     } else if (state.initialized) {
       await state.resetWithStartingPoint(createdAt);
       setTimeout(() => {
-        const topOffset = getMessageTopOffset(createdAt);
+        // NOTE: Inside setTimeout callback, the closure-captured `state` is a stale snapshot (before reset).
+        // Use store.getState() to get the latest state (including updated messages) at callback execution time.
+        const currentState = store.getState();
+
+        if (currentState.messages.length === 0) return; // empty channel case.
+        let message = null;
+        let distance = Number.MAX_SAFE_INTEGER;
+        for (const it of currentState.messages) {
+          if (it.createdAt > createdAt) break;
+          const diff = Math.abs(it.createdAt - createdAt);
+          if (diff < distance) {
+            distance = diff;
+            message = it;
+          }
+        }
+
+        const topOffset = getMessageTopOffset(message.createdAt);
         if (topOffset !== null) {
           state.scrollPubSub.publish('scroll', {
             top: topOffset,