fix: scroll AI chat to bottom on side panel reopen (#20413)

## Summary
- Fix: when reopening the AI chat side panel, users landed at the top of
the conversation and had to scroll down to find the latest messages
- Root cause: the side panel fully unmounts on close
([SidePanelForDesktop.tsx](packages/twenty-front/src/modules/side-panel/components/SidePanelForDesktop.tsx)
clears `shouldShowContent` after the close transition), so on reopen the
scroll wrapper is recreated with `scrollTop = 0`. The existing
initial-scroll-to-bottom only fires on thread change, but the
displayed-thread atom outlives the unmount, so no thread change is
detected on a remount and the scroll-to-bottom never runs
- Fix: add a tiny `AgentChatScrollToBottomOnMountLayoutEffect` rendered
inside the message list that calls `scrollAiChatToBottom()` directly in
`useLayoutEffect`. Because the parent returns `null` when there are no
messages, the mount only fires when there is content to scroll past

## Why direct scroll, not the existing flag
`agentChatIsInitialScrollPendingOnThreadChangeState` is paired with a
`MutationObserver` settle that only clears the flag once the subtree is
quiet for 150 ms. During a live stream the message subtree mutates on
every token, the settle resets indefinitely and `visibility: hidden`
never lifts. The thread-change handler avoids this because it is gated
on `!agentChatIsStreaming`
([AgentChatStreamSubscriptionEffect.tsx:78-79](packages/twenty-front/src/modules/ai/components/AgentChatStreamSubscriptionEffect.tsx)),
but a mount can happen at any time, including mid-stream. Scrolling the
DOM directly in `useLayoutEffect` runs synchronously between commit and
paint, so the user sees the bottom on the first paint with no flash and
no settle dependency.

## Tradeoff
A user who scrolled up to read history and then closes/reopens the panel
will land back at the bottom instead of where they were. Standard chat
UX (Slack, ChatGPT, iMessage); preserving per-thread scroll position
would need a new atom and is left out of scope.

## Test plan
- [ ] Open AI chat with messages, close the side panel, reopen → lands
at the bottom (latest messages visible)
- [ ] Reopen the side panel **mid-stream** → lands at the bottom and
continues to follow new tokens (chat is not hidden)
- [ ] Switch between threads → existing thread-change scroll still works
(no double-scroll, no regression)
- [ ] Open AI chat with no messages → no flash, no errors
- [ ] Rapidly close/reopen the panel a few times → each reopen lands at
the bottom

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/twenty-front/src/modules/ai/components/AgentChatScrollToBottomOnMountLayoutEffect.tsx

@@ -0,0 +1,10 @@
+import { scrollAiChatToBottom } from '@/ai/utils/scrollAiChatToBottom';
+import { useLayoutEffect } from 'react';
+
+export const AgentChatScrollToBottomOnMountLayoutEffect = () => {
+  useLayoutEffect(() => {
+    scrollAiChatToBottom();
+  }, []);
+
+  return null;
+};

packages/twenty-front/src/modules/ai/components/AiChatTabMessageList.tsx

@@ -3,6 +3,7 @@ import { AiChatLastMessageWithStreamingState } from '@/ai/components/AiChatLastM
 import { AiChatNonLastMessageIdsList } from '@/ai/components/AiChatNonLastMessageIdsList';
 import { AiChatScrollToBottomButton } from '@/ai/components/AiChatScrollToBottomButton';
 import { AgentChatScrollToBottomOnDisplayedThreadChangeLayoutEffect } from '@/ai/components/AgentChatScrollToBottomOnDisplayedThreadChangeLayoutEffect';
+import { AgentChatScrollToBottomOnMountLayoutEffect } from '@/ai/components/AgentChatScrollToBottomOnMountLayoutEffect';
 import { AI_CHAT_SCROLL_WRAPPER_ID } from '@/ai/constants/AiChatScrollWrapperId';
 import { agentChatHasMessageComponentSelector } from '@/ai/states/selectors/agentChatHasMessageComponentSelector';
 import { agentChatIsInitialScrollPendingOnThreadChangeState } from '@/ai/states/agentChatIsInitialScrollPendingOnThreadChangeState';
@@ -49,6 +50,7 @@ export const AiChatTabMessageList = () => {
         <AiChatLastMessageWithStreamingState />
         <AiChatErrorUnderMessageList />
         <AgentChatScrollToBottomOnDisplayedThreadChangeLayoutEffect />
+        <AgentChatScrollToBottomOnMountLayoutEffect />
       </ScrollWrapper>
       <AiChatScrollToBottomButton />
     </StyledScrollWrapperContainer>