diff --git a/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java b/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
index f5895ffdb207..0a5a74b49c65 100644
--- a/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
+++ b/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
@@ -47,17 +47,25 @@
 import org.apache.beam.sdk.coders.MapCoder;
 import org.apache.beam.sdk.coders.StringUtf8Coder;
 import org.apache.beam.sdk.coders.VarIntCoder;
+import org.apache.beam.sdk.io.GenerateSequence;
+import org.apache.beam.sdk.state.TimeDomain;
+import org.apache.beam.sdk.state.Timer;
+import org.apache.beam.sdk.state.TimerSpec;
+import org.apache.beam.sdk.state.TimerSpecs;
 import org.apache.beam.sdk.testing.LargeKeys;
 import org.apache.beam.sdk.testing.NeedsRunner;
 import org.apache.beam.sdk.testing.PAssert;
 import org.apache.beam.sdk.testing.TestPipeline;
 import org.apache.beam.sdk.testing.TestStream;
 import org.apache.beam.sdk.testing.UsesTestStreamWithProcessingTime;
+import org.apache.beam.sdk.testing.UsesTimersInParDo;
+import org.apache.beam.sdk.testing.UsesUnboundedPCollections;
 import org.apache.beam.sdk.testing.ValidatesRunner;
 import org.apache.beam.sdk.transforms.display.DisplayData;
 import org.apache.beam.sdk.transforms.windowing.AfterPane;
 import org.apache.beam.sdk.transforms.windowing.AfterProcessingTime;
 import org.apache.beam.sdk.transforms.windowing.AfterWatermark;
+import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
 import org.apache.beam.sdk.transforms.windowing.FixedWindows;
 import org.apache.beam.sdk.transforms.windowing.GlobalWindow;
 import org.apache.beam.sdk.transforms.windowing.IntervalWindow;
@@ -90,7 +98,8 @@
 /** Tests for GroupByKey. */
 @SuppressWarnings({
   "rawtypes", // TODO(https://issues.apache.org/jira/browse/BEAM-10556)
-  "unchecked"
+  "unchecked",
+  "unused"
 })
 @RunWith(Enclosed.class)
 public class GroupByKeyTest implements Serializable {
@@ -103,7 +112,7 @@ public abstract static class SharedTestBase {
 
   /** Tests validating basic {@link GroupByKey} scenarios. */
   @RunWith(JUnit4.class)
-  public static class BasicTests extends SharedTestBase {
+  public static class BasicTests extends SharedTestBase implements Serializable {
     @Test
     @Category(ValidatesRunner.class)
     public void testGroupByKey() {
@@ -186,6 +195,109 @@ public void testCombiningAccumulatingProcessingTime() throws Exception {
       p.run();
     }
 
+    /**
+     * Tests that data from a processing time trigger flows through subsequent GroupByKey
+     * transforms. To test this with TestStream, we check that it arrives in an early pane,
+     * demonstrating that the watermark did not cause the output.
+     */
+    @Test
+    @Category({ValidatesRunner.class, UsesTestStreamWithProcessingTime.class})
+    public void testAfterProcessingTimeContinuationTriggerEarly() throws Exception {
+      final long triggerMillis = 1; // Setting of the processing time trigger
+      final long advanceMillis = 10; // How far we advance time for the first GBK to trigger
+      final long waitMillis = 500; // How far we advance time for the second GBK to trigger
+
+      PCollection<Integer> triggeredSums =
+          p.apply(
+                  TestStream.create(VarIntCoder.of())
+                      .advanceWatermarkTo(new Instant(0))
+                      .addElements(42)
+                      .advanceProcessingTime(Duration.millis(advanceMillis))
+                      .advanceProcessingTime(Duration.millis(waitMillis))
+                      .advanceWatermarkToInfinity())
+              .apply(
+                  Window.<Integer>configure()
+                      .triggering(
+                          Repeatedly.forever(
+                              AfterProcessingTime.pastFirstElementInPane()
+                                  .plusDelayOf(Duration.millis(triggerMillis))))
+                      .accumulatingFiredPanes()
+                      .withOnTimeBehavior(Window.OnTimeBehavior.FIRE_IF_NON_EMPTY)
+                      .withAllowedLateness(Duration.millis(0)))
+              .apply("Triggered sum", Sum.integersGlobally().withoutDefaults())
+              .apply("Second Triggered sum", Sum.integersGlobally().withoutDefaults());
+
+      PAssert.that(triggeredSums).inEarlyGlobalWindowPanes().containsInAnyOrder(42);
+
+      p.run();
+    }
+
+    /**
+     * Tests that data from a processing time trigger flows through subsequent GroupByKey
+     * transforms. This version does not depend on {@link TestStream} with processing time, because
+     * many runners do not support it.
+     *
+     * <p>Since a delay in processing time is fundamental to this test, it unfortunately must have a
+     * real-time sleep. Currently the sleep is sub-second so it is acceptable delay for a unit test
+     * suite.
+     */
+    @Test
+    @Category({ValidatesRunner.class, UsesTimersInParDo.class, UsesUnboundedPCollections.class})
+    public void testAfterProcessingTimeContinuationTriggerUsingState() throws Exception {
+      final long triggerMillis = 1;
+      final long waitMillis = 500;
+
+      PCollection<Integer> triggeredSums =
+          p.apply(
+                  GenerateSequence.from(0)
+                      .to(1)) // forces unbounded so delay cannot be fast-forwarded
+              .apply(WithKeys.of("dummy key"))
+              .apply(
+                  "output then delay",
+                  ParDo.of(
+                      new DoFn<KV<String, Long>, Integer>() {
+                        private static final String DELAY_TIMER = "delay";
+
+                        @TimerId(DELAY_TIMER)
+                        private final TimerSpec delayTimerSpec =
+                            TimerSpecs.timer(TimeDomain.EVENT_TIME);
+
+                        @ProcessElement
+                        public void process(
+                            @Timestamp Instant timestamp,
+                            BoundedWindow window,
+                            OutputReceiver<Integer> out,
+                            @TimerId(DELAY_TIMER) Timer delayTimer) {
+                          out.output(42);
+
+                          // wait a little bit while stopping downstream from firing the window
+                          delayTimer.set(window.maxTimestamp().minus(Duration.millis(10)));
+                        }
+
+                        @OnTimer(DELAY_TIMER)
+                        public void onDelay(@Timestamp Instant timestamp)
+                            throws InterruptedException {
+                          // noop, just here to force the pipeline to sleep so downstream GBK will
+                          // trigger
+                          Thread.sleep(waitMillis);
+                        }
+                      }))
+              .apply(
+                  Window.<Integer>configure()
+                      .triggering(
+                          Repeatedly.forever(
+                              AfterProcessingTime.pastFirstElementInPane()
+                                  .plusDelayOf(Duration.millis(triggerMillis))))
+                      .accumulatingFiredPanes()
+                      .withAllowedLateness(Duration.millis(0)))
+              .apply("Triggered sum", Sum.integersGlobally().withoutDefaults())
+              .apply("Second Triggered sum", Sum.integersGlobally().withoutDefaults());
+
+      PAssert.that(triggeredSums).inEarlyGlobalWindowPanes().containsInAnyOrder(42);
+
+      p.run();
+    }
+
     @Test
     public void testGroupByKeyNonDeterministic() throws Exception {