qt tx notifications: wait until sync finishes

Comment is no longer relevant. Also, actually it was incorrect.
Each txn is only downloaded once, though 'added' multiple times to the wallet.
The triggers are only sent out by the Synchronizer, once, when downloaded.
The actual reason for the inconsistency was that get_wallet_delta can only
give complete results once the wallet is synced.

electrum/gui/qt/main_window.py

@@ -588,11 +588,10 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         self.show_message(msg, title="Electrum - " + _("Reporting Bugs"))
 
     def notify_transactions(self):
-        # note: during initial history sync for a wallet, many txns will be
-        # received multiple times. hence the "total amount received" can be
-        # a lot different than should be. this is expected though not intended
         if self.tx_notification_queue.qsize() == 0:
             return
+        if not self.wallet.up_to_date:
+            return  # no notifications while syncing
         now = time.time()
         rate_limit = 20  # seconds
         if self.tx_notification_last_time + rate_limit > now: