WindowIterator should emit items in order when scrolling backwards.

Closes #2857
Original pull request: #2858

Author: christophstrobl

src/main/java/org/springframework/data/support/WindowIterator.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.data.support;
 
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
 import java.util.function.Function;
@@ -81,7 +82,9 @@ public boolean hasNext() {
 
 			if (currentIterator == null) {
 				if (currentWindow != null) {
-					currentIterator = currentWindow.iterator();
+					currentIterator = isBackwardsScrolling(currentPosition)
+							? currentWindow.stream().sorted(Collections.reverseOrder()).iterator()
+							: currentWindow.iterator();
 				}
 			}
 
@@ -116,15 +119,17 @@ public T next() {
 
 	private static ScrollPosition getNextPosition(ScrollPosition currentPosition, Window<?> window) {
 
-		if (currentPosition instanceof KeysetScrollPosition ksp) {
-			if (ksp.scrollsBackward()) {
-				return window.positionAt(0);
-			}
+		if (isBackwardsScrolling(currentPosition)) {
+			return window.positionAt(0);
 		}
 
 		return window.positionAt(window.size() - 1);
 	}
 
+	private static boolean isBackwardsScrolling(ScrollPosition position) {
+		return position instanceof KeysetScrollPosition ksp ? ksp.scrollsBackward() : false;
+	}
+
 	/**
 	 * Builder API to construct a {@link WindowIterator}.
 	 *

src/test/java/org/springframework/data/domain/WindowIteratorUnitTests.java

@@ -131,7 +131,7 @@ void allowsToIterateAllWindows() {
 		assertThat(capturedResult).containsExactly("a", "b", "c", "d");
 	}
 
-	@Test // GH-2151
+	@Test // GH-2151, GH-2857
 	void considersBackwardKeysetScrolling() {
 
 		Window<String> initial = Window.from(List.of("c", "d"),
@@ -152,6 +152,6 @@ void considersBackwardKeysetScrolling() {
 		}).startingAt(ScrollPosition.keyset().backward());
 
 		List<String> items = Streamable.of(() -> iterator).toList();
-		assertThat(items).containsExactly("c", "d", "a", "b");
+		assertThat(items).containsExactly("d", "c", "b", "a");
 	}
 }