fix: Don't recreate JS TreeTable subscriptions when viewport changes

Resolves an issue where subscription recreation would release an export
ticket that was still needed for the next subscription.

Also improves tree viewport performance by sending changes instead of
waiting for a new subscription to exist.

Fixes #6053

web/client-api/src/main/java/io/deephaven/web/client/api/tree/JsTreeTable.java

@@ -599,14 +599,25 @@ private void replaceSubscription(RebuildStep step) {
                     viewTicket.release();
                     viewTicket = null;
                 }
-            case SUBSCRIPTION:
+
+                // In all of the above cases, we replace the subscription
                 if (stream != null) {
                     stream.then(stream -> {
                         stream.close();
                         return null;
                     });
                     stream = null;
                 }
+                break;
+            case SUBSCRIPTION:
+                // If it exists, adjust the existing subscription, otherwise create a new one
+                if (stream != null) {
+                    stream.then(subscription -> {
+                        subscription.setViewport(firstRow, lastRow, Js.uncheckedCast(columns), (double) updateInterval);
+                        return null;
+                    });
+                    return;
+                }
         }
 
         Promise<TreeSubscription> stream = Promise.resolve(defer())