public void onPeriodicEmit() {
        updateCombinedWatermark();
    }
@Test
    void singleDeferredIdleness() {
        TestingWatermarkOutput underlyingWatermarkOutput = createTestingWatermarkOutput();
        WatermarkOutputMultiplexer multiplexer =
                new WatermarkOutputMultiplexer(underlyingWatermarkOutput);

        WatermarkOutput watermarkOutput = createDeferredOutput(multiplexer);

        watermarkOutput.markIdle();
        multiplexer.onPeriodicEmit();

        assertThat(underlyingWatermarkOutput.lastWatermark()).isNull();
        assertThat(underlyingWatermarkOutput.isIdle()).isTrue();
    }