private WindowPane<K, Object> computePaneOutput(TriggerKey<K> triggerKey, Object windowVal) {
    WindowKey<K> windowKey = new WindowKey(triggerKey.getKey(), Long.toString(triggerKey.getTimestamp()));
    WindowPane<K, Object> paneOutput =
        new WindowPane<>(windowKey, windowVal, window.getAccumulationMode(), triggerKey.getType());
    LOG.trace("Emitting pane output for trigger key {}", triggerKey);
    return paneOutput;
  }
Computes the pane output corresponding to a {@link TriggerKey} that fired.
samza-core/src/main/java/org/apache/samza/operators/impl/WindowOperatorImpl.java