fix: resolve CRDT collaboration sync by registering broadcast listener before channel subscribe

The yjs-update broadcast listener was added after the Supabase channel
was already subscribed, which meant it never received messages. Moved
the listener registration to the builder chain before .subscribe()
(matching how cursor/node-lock listeners work), and removed the broken
isRemoteUpdateRef guard that caused ReferenceErrors preventing local
changes from reaching the CRDT.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/editor/[projectId]/FlowchartEditor.tsx

@@ -527,6 +527,12 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
 
   const [characters, setCharacters] = useState<Character[]>(migratedData.characters)
   const [variables, setVariables] = useState<Variable[]>(migratedData.variables)
+
+  // Refs to always have the latest characters/variables for the CRDT persist callback
+  const charactersRef = useRef(characters)
+  const variablesRef = useRef(variables)
+  charactersRef.current = characters
+  variablesRef.current = variables
   const [showSettings, setShowSettings] = useState(false)
   const [showShare, setShowShare] = useState(false)
   const [showHistory, setShowHistory] = useState(false)
@@ -540,7 +546,6 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
   const realtimeRef = useRef<RealtimeConnection | null>(null)
   const crdtRef = useRef<CRDTManager | null>(null)
   const auditRef = useRef<AuditTrailRecorder | null>(null)
-  const isRemoteUpdateRef = useRef(false)
   const cursorThrottleRef = useRef<number>(0)
   const [collaborationNotifications, setCollaborationNotifications] = useState<CollaborationNotification[]>([])
   const [nodeLocks, setNodeLocks] = useState<Map<string, NodeLockInfo>>(new Map())
@@ -554,14 +559,10 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
 
     const crdtManager = new CRDTManager({
       onNodesChange: (crdtNodes: FlowchartNode[]) => {
-        isRemoteUpdateRef.current = true
         setNodes(toReactFlowNodes(crdtNodes))
-        isRemoteUpdateRef.current = false
       },
       onEdgesChange: (crdtEdges: FlowchartEdge[]) => {
-        isRemoteUpdateRef.current = true
         setEdges(toReactFlowEdges(crdtEdges))
-        isRemoteUpdateRef.current = false
       },
       onPersist: async (persistNodes: FlowchartNode[], persistEdges: FlowchartEdge[]) => {
         try {
@@ -571,8 +572,8 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
               flowchart_data: {
                 nodes: persistNodes,
                 edges: persistEdges,
-                characters,
+                characters: charactersRef.current,
-                variables,
+                variables: variablesRef.current,
               },
             })
             .eq('id', projectId)
@@ -633,6 +634,9 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
           return next
         })
       },
+      onCRDTUpdate: (update: number[]) => {
+        crdtManager.applyRemoteUpdate(update)
+      },
       onChannelSubscribed: (channel) => {
         crdtManager.connectChannel(channel)
       },
@@ -678,7 +682,6 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
   }, [edges])
 
   useEffect(() => {
-    if (isRemoteUpdateRef.current) return
     crdtRef.current?.updateNodes(nodesForCRDT)
     if (!isRevertingRef.current) {
       auditRef.current?.recordNodeChanges(nodesForCRDT)
@@ -686,7 +689,6 @@ function FlowchartEditorInner({ projectId, projectName, userId, userDisplayName,
   }, [nodesForCRDT])
 
   useEffect(() => {
-    if (isRemoteUpdateRef.current) return
     crdtRef.current?.updateEdges(edgesForCRDT)
     if (!isRevertingRef.current) {
       auditRef.current?.recordEdgeChanges(edgesForCRDT)

src/lib/collaboration/crdt.ts

@@ -57,25 +57,23 @@ export class CRDTManager {
     }, 'init')
   }
 
-  /** Connect to a Supabase Realtime channel for syncing updates */
+  /** Connect to a Supabase Realtime channel for outbound broadcasts */
   connectChannel(channel: RealtimeChannel): void {
     this.channel = channel
+  }
 
-    // Listen for broadcast updates from other clients
+  /** Apply a remote CRDT update received via broadcast */
-    channel.on('broadcast', { event: BROADCAST_EVENT }, (payload) => {
+  applyRemoteUpdate(updateData: number[]): void {
     if (this.isDestroyed) return
-      const data = payload.payload as { update?: number[] } | undefined
+    const update = new Uint8Array(updateData)
-      if (data?.update) {
-        const update = new Uint8Array(data.update)
     this.isApplyingRemote = true
     Y.applyUpdate(this.doc, update, 'remote')
     this.isApplyingRemote = false
     // Notify React state of remote changes
     this.notifyNodesChange()
     this.notifyEdgesChange()
-        this.schedulePersist()
+    // Note: we do NOT schedulePersist here. Only the originating client
-      }
+    // persists its own changes to avoid write races and stale data overwrites.
-    })
   }
 
   /** Apply local node changes to the Yjs document */

src/lib/collaboration/realtime.ts

@@ -35,6 +35,7 @@ type RealtimeCallbacks = {
   onChannelSubscribed?: (channel: RealtimeChannel) => void
   onCursorUpdate?: (cursor: RemoteCursor) => void
   onNodeLockUpdate?: (lock: NodeLock | null, userId: string) => void
+  onCRDTUpdate?: (update: number[]) => void
 }
 
 const HEARTBEAT_INTERVAL_MS = 30_000
@@ -131,6 +132,11 @@ export class RealtimeConnection {
           this.callbacks.onNodeLockUpdate?.(null, payload.userId)
         }
       })
+      .on('broadcast', { event: 'yjs-update' }, ({ payload }) => {
+        if (payload?.update) {
+          this.callbacks.onCRDTUpdate?.(payload.update)
+        }
+      })
       .subscribe(async (status) => {
         if (this.isDestroyed) return