fix: Fix regression when sync id check is disabled

flow-server/src/main/java/com/vaadin/flow/server/communication/LongPollingCacheFilter.java

@@ -76,7 +76,10 @@ public BroadcastAction filter(String broadcasterId, AtmosphereResource r,
             String uuid = r.uuid();
             int lastSeenOnClient = session.getAttribute(SEEN_SERVER_SYNC_ID,
                     Integer.class);
-            if (pushMessage.alreadySeen(lastSeenOnClient)) {
+            if (lastSeenOnClient == -1) {
+                return new BroadcastAction(BroadcastAction.ACTION.CONTINUE,
+                        message);
+            } else if (pushMessage.alreadySeen(lastSeenOnClient)) {
                 getLogger().trace(
                         "Discarding message {} for resource {} as client already seen {}. {}",
                         pushMessage.serverSyncId, uuid, lastSeenOnClient,

flow-server/src/test/java/com/vaadin/flow/server/communication/LongPollingCacheFilterTest.java

@@ -81,6 +81,19 @@ public void filter_notLongPollingTransport_continueWithCurrentMessage() {
         verifyMessageIsNotCached();
     }
 
+    @Test
+    public void filter_syncIdCheckDisabled_continueWithCurrentMessage() {
+        setTransport(AtmosphereResource.TRANSPORT.LONG_POLLING);
+        setSeenServerSyncIdHeader(-1);
+        BroadcastAction action = filter.filter("broadcasterId", resource,
+                originalMessage, message);
+        Assert.assertEquals(ACTION.CONTINUE, action.action());
+        Assert.assertSame(
+                "Message should not be altered by filter if server sync id header is missing",
+                message, action.message());
+        verifyMessageIsNotCached();
+    }
+
     @Test
     public void filter_missingLastSeenServerSyncId_continueWithCurrentMessage() {
         setTransport(AtmosphereResource.TRANSPORT.LONG_POLLING);
@@ -224,4 +237,4 @@ private void simulatePushConnection() {
     private void verifyMessageIsNotCached() {
         Mockito.verifyNoInteractions(cache);
     }
-}
+}