fixup! [FLINK-19109][task] Ignore isLoopRunning in MailboxExecutor.is…

…Idle

flink-streaming-java/src/test/java/org/apache/flink/streaming/runtime/tasks/mailbox/MailboxExecutorImplTest.java

@@ -79,6 +79,26 @@ public void tearDown() {
 		}
 	}
 
+	@Test
+	public void testIsIdle() throws Exception {
+		MailboxProcessor processor = new MailboxProcessor(MailboxDefaultAction.Controller::suspendDefaultAction);
+		MailboxExecutorImpl executor = (MailboxExecutorImpl) processor.getMailboxExecutor(DEFAULT_PRIORITY);
+
+		assertFalse(executor.isIdle());
+
+		processor.runMailboxStep(); // suspend default action after suspension
+		processor.mailbox.drain(); // drop any control mails
+
+		assertTrue(executor.isIdle());
+
+		executor.execute(() -> {}, "");
+		assertFalse(executor.isIdle());
+
+		processor.mailbox.drain();
+		processor.mailbox.quiesce();
+		assertFalse(executor.isIdle());
+	}
+
 	@Test
 	public void testOperations() throws Exception {
 		AtomicBoolean wasExecuted = new AtomicBoolean(false);