Message List: Suppress spurious infinite scroll triggers on Safari

In Safari the `IntersectionObserver` can deliver entries computed between
DOM mutation and the deferred scroll restore, causing the history trigger
to fire repeatedly and the viewport to jump.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/middle/MessageList.tsx

@@ -271,6 +271,9 @@ const MessageList = ({
   const memoUnreadDividerBeforeIdRef = useRef<number | undefined>();
   const memoFocusingIdRef = useRef<number>();
   const isScrollTopJustUpdatedRef = useRef(false);
+  // Suppresses spurious load-more triggers caused by Safari delivering stale
+  // `IntersectionObserver` entries between DOM mutation and scroll restore
+  const isReplacingHistoryRef = useRef(false);
   const shouldAnimateAppearanceRef = useRef(Boolean(lastMessage));
   const scrollSnapDisabledTimerRef = useRef<number>();
   const typingDraftSnapTriggeredIdRef = useRef<number>();
@@ -636,7 +639,10 @@ const MessageList = ({
   });
 
   useSyncEffect(
-    () => forceMeasure(() => rememberScrollPositionRef.current()),
+    () => {
+      isReplacingHistoryRef.current = true;
+      forceMeasure(() => rememberScrollPositionRef.current());
+    },
     // This will run before modifying content and should match deps for `useLayoutEffectWithPrevDeps` below
     [messageIds, isViewportNewest, rememberScrollPositionRef],
   );
@@ -770,6 +776,9 @@ const MessageList = ({
 
       return () => {
         resetScroll(container, Math.ceil(newScrollTop));
+        requestMeasure(() => {
+          isReplacingHistoryRef.current = false;
+        });
         restartCurrentScrollAnimation();
 
         scrollOffsetRef.current = Math.max(Math.ceil(scrollHeight - newScrollTop), offsetHeight);
@@ -907,6 +916,7 @@ const MessageList = ({
         anchorIdRef={anchorIdRef}
         memoUnreadDividerBeforeIdRef={memoUnreadDividerBeforeIdRef}
         memoFirstUnreadIdRef={memoFirstUnreadIdRef}
+        isReplacingHistoryRef={isReplacingHistoryRef}
         threadId={threadId}
         type={type}
         isReady={isReady}

src/components/middle/MessageListContent.tsx

@@ -69,6 +69,7 @@ interface OwnProps {
   anchorIdRef: { current: string | undefined };
   memoUnreadDividerBeforeIdRef: { current: number | undefined };
   memoFirstUnreadIdRef: { current: number | undefined };
+  isReplacingHistoryRef: { current: boolean };
   type: MessageListType;
   isReady: boolean;
   hasLinkedChat: boolean | undefined;
@@ -109,6 +110,7 @@ const MessageListContent = ({
   anchorIdRef,
   memoUnreadDividerBeforeIdRef,
   memoFirstUnreadIdRef,
+  isReplacingHistoryRef,
   type,
   isReady,
   hasLinkedChat,
@@ -164,6 +166,7 @@ const MessageListContent = ({
     isViewportNewest,
     isUnread,
     isReady,
+    isReplacingHistoryRef,
     onScrollDownToggle,
     onNotchToggle,
   });

src/components/middle/hooks/useScrollHooks.ts

@@ -32,6 +32,7 @@ export default function useScrollHooks({
   isViewportNewest,
   isUnread,
   isReady,
+  isReplacingHistoryRef,
   onScrollDownToggle,
   onNotchToggle,
 }: {
@@ -42,6 +43,7 @@ export default function useScrollHooks({
   isViewportNewest: boolean;
   isUnread: boolean;
   isReady: boolean;
+  isReplacingHistoryRef: { current: boolean };
   onScrollDownToggle: BooleanToVoidFunction | undefined;
   onNotchToggle: AnyToVoidFunction | undefined;
 }) {
@@ -109,6 +111,10 @@ export default function useScrollHooks({
       return;
     }
 
+    if (isReplacingHistoryRef.current) {
+      return;
+    }
+
     entries.forEach(({ isIntersecting, target }) => {
       if (!isIntersecting) return;