Fix positionFunction created by ScrollDelegate

Before this commit, positionFunction created by ScrollDelegate retains incorrect state when perform backward scrolling.
1. We must use window.positionAt(1) instead of window.positionAt(0) if we want position at the begin of window, we should call getResultWindow() first.
2. The direction always be FORWARD, it should be BACKWARD if the original direction is BACKWARD.

Closes spring-projectsGH-2999

Author: quaff

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/query/ScrollDelegate.java

@@ -35,6 +35,7 @@
  * Delegate to run {@link ScrollPosition scroll queries} and create result {@link Window}.
  *
  * @author Mark Paluch
+ * @author Yanming Zhou
  * @since 3.1
  */
 public class ScrollDelegate<T> {
@@ -80,17 +81,17 @@ private static <T> Window<T> createWindow(Sort sort, int limit, Direction direct
 			JpaEntityInformation<T, ?> entity, List<T> result) {
 
 		KeysetScrollDelegate delegate = KeysetScrollDelegate.of(direction);
-		List<T> resultsToUse = delegate.postProcessResults(result);
+		List<T> resultsToUse = delegate.getResultWindow(delegate.postProcessResults(result), limit);
 
 		IntFunction<ScrollPosition> positionFunction = value -> {
 
-			T object = result.get(value);
+			T object = resultsToUse.get(value);
 			Map<String, Object> keys = entity.getKeyset(sort.stream().map(Order::getProperty).toList(), object);
 
-			return ScrollPosition.forward(keys);
+			return ScrollPosition.of(keys, direction);
 		};
 
-		return Window.from(delegate.getResultWindow(resultsToUse, limit), positionFunction, hasMoreElements(result, limit));
+		return Window.from(resultsToUse, positionFunction, hasMoreElements(result, limit));
 	}
 
 	private static <T> Window<T> createWindow(List<T> result, int limit,