fix: forking of a regeneration sometimes copied messages after the root

server/api/topic/[topicId]/fork.post.ts

@@ -85,17 +85,23 @@ export default defineEventHandler(async (event) => {
         current = current.parentMessageId ? messageMap.get(current.parentMessageId) : undefined;
     }
 
-    const forkIndex = topicMessages.findIndex(m => m.id === messageId);
+    // A regeneration can be created after later roots, so the fork
+    // point has to be located by its root rather than by its own createdAt.
+    let forkRoot = forkMessage;
+    while (forkRoot.parentMessageId && messageMap.has(forkRoot.parentMessageId)) {
+        forkRoot = messageMap.get(forkRoot.parentMessageId)!;
+    }
+
+    const rootMessages = topicMessages.filter(m => !m.parentMessageId);
+    const forkIndex = rootMessages.findIndex(m => m.id === forkRoot.id);
     for (let i = 0; i <= forkIndex; i++) {
-        const msg = topicMessages[i]!;
+        const msg = rootMessages[i]!;
-        if (!msg.parentMessageId) {
         messageIdsToCopy.add(msg.id);
         const children = childrenMap.get(msg.id) || [];
         for (const child of children) {
             messageIdsToCopy.add(child.id);
         }
     }
-    }
 
     const addDescendants = (msgId: string) => {
         const children = childrenMap.get(msgId) || [];