Work around wrench deadlock when a transaction scrolls.

wrench/src/wrench.rs

@@ -512,15 +512,31 @@ impl Wrench {
                 false,
             );
         }
+        // TODO(nical) - Need to separate the set_display_list from the scrolling
+        // operations into separate transactions for mysterious -but probably related
+        // to the other comment below- reasons.
+        self.api.send_transaction(self.document_id, txn);
 
+        let mut txn = Transaction::new();
         for (id, offset) in scroll_offsets {
             txn.scroll_node_with_id(
                 *offset,
                 *id,
                 ScrollClamping::NoClamping,
             );
         }
+        // TODO(nical) - Wrench does not notify frames when there was scrolling
+        // in the transaction (See RenderNotifier implementations). If we don't
+        // generate a frame after scrolling, wrench just stops and some tests
+        // will time out.
+        // I suppose this was to avoid taking the snapshot after scrolling if
+        // there was other updates coming in a subsequent messages but it's very
+        // error-prone with transactions.
+        // For now just send two transactions to avoid the deadlock, but we should
+        // figure this out.
+        self.api.send_transaction(self.document_id, txn);
 
+        let mut txn = Transaction::new();
         txn.generate_frame();
         self.api.send_transaction(self.document_id, txn);
     }