Keep selected activity history stable

frontend/app/components/assistant-ui/activity-panel.tsx

@@ -168,21 +168,32 @@ export function ActivityPanel() {
 		() => collectLiveRunItems(liveRunStatus),
 		[liveRunStatus],
 	);
+	const selectedMessageId = selectedId ? activityMessageId(selectedId) : null;
+	const selectedMessageItems = selectedMessageId
+		? messageItems.filter(
+				(item) => activityMessageId(item.id) === selectedMessageId,
+			)
+		: [];
 	const shouldUseLiveItems =
+		!selectedMessageId &&
 		liveItems.length > 0 &&
 		activeSessionId !== null &&
 		(activeSessionId !== replayedSessionId || hasActiveRun(liveRunStatus));
 	const items = shouldUseLiveItems ? liveItems : messageItems;
-	const selectedMessageId = selectedId ? activityMessageId(selectedId) : null;
 	const selectedItems = selectedMessageId
 		? items.filter((item) => activityMessageId(item.id) === selectedMessageId)
 		: [];
 	const visibleItems =
-		selectedMessageId && selectedItems.length > 0 ? selectedItems : items;
+		selectedMessageId && selectedMessageItems.length > 0
+			? selectedMessageItems
+			: selectedMessageId && selectedItems.length > 0
+				? selectedItems
+				: items;
 	const selectedActivityMissing = Boolean(
 		selectedMessageId &&
 			!shouldUseLiveItems &&
-			items.length > 0 &&
+			messageItems.length > 0 &&
+			selectedMessageItems.length === 0 &&
 			selectedItems.length === 0,
 	);
 	const prevLatestIdRef = useRef<string | null>(null);
@@ -207,13 +218,14 @@ export function ActivityPanel() {
 		const latestId = items.at(-1)?.id ?? null;
 		if (
 			mode === "activity" &&
+			!selectedId &&
 			latestId &&
 			latestId !== prevLatestIdRef.current
 		) {
 			openActivity();
 		}
 		prevLatestIdRef.current = latestId;
-	}, [items, mode, openActivity]);
+	}, [items, mode, openActivity, selectedId]);
 
 	if (!open) return null;