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

        WatermarkOutput watermarkOutput = createDeferredOutput(multiplexer);

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

        assertThat(underlyingWatermarkOutput.isIdle()).isTrue();

        watermarkOutput.emitWatermark(new Watermark(0));
        multiplexer.onPeriodicEmit();

        assertThat(underlyingWatermarkOutput.lastWatermark()).isEqualTo(new Watermark(0));
        assertThat(underlyingWatermarkOutput.isIdle()).isFalse();
    }