Align OffsetScrolling to zero-based indexes.

Closes #1764

Author: mp911de

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/support/FetchableFluentQueryByExample.java

@@ -102,7 +102,11 @@ public Window<R> scroll(ScrollPosition scrollPosition) {
 
 		if (scrollPosition instanceof OffsetScrollPosition osp) {
 
-			Query query = createQuery().sort(getSort()).offset(osp.getOffset());
+			Query query = createQuery().sort(getSort());
+
+			if (!osp.isInitial()) {
+				query = query.offset(osp.getOffset() + 1);
+			}
 
 			if (getLimit() > 0) {
 				query = query.limit(getLimit());

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/support/ScrollDelegate.java

@@ -54,7 +54,7 @@ public static <T> Window<T> scroll(Query query, Function<Query, List<T>> queryFu
 		List<T> result = queryFunction.apply(query);
 
 		if (scrollPosition instanceof OffsetScrollPosition offset) {
-			return createWindow(result, limit, OffsetScrollPosition.positionFunction(offset.getOffset()));
+			return createWindow(result, limit, offset.positionFunction());
 		}
 
 		throw new UnsupportedOperationException("ScrollPosition " + scrollPosition + " not supported");

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryIntegrationTests.java

@@ -1137,7 +1137,7 @@ void findByScrollPosition() {
 		assertThat(first.map(DummyEntity::getName)).containsExactly("one", "three");
 
 		Window<DummyEntity> second = repository.findBy(example, q -> q.limit(2).sortBy(Sort.by("name")))
-				.scroll(ScrollPosition.offset(2));
+				.scroll(ScrollPosition.offset(1));
 		assertThat(second.map(DummyEntity::getName)).containsExactly("two");
 
 		WindowIterator<DummyEntity> iterator = WindowIterator.of(

spring-data-r2dbc/src/main/java/org/springframework/data/r2dbc/repository/support/SimpleR2dbcRepository.java

@@ -409,7 +409,11 @@ public Mono<Window<T>> scroll(ScrollPosition scrollPosition) {
 				int limit = getLimit();
 				return createQuery(q -> {
 
-					Query queryToUse = q.offset(osp.getOffset());
+					Query queryToUse = q;
+
+					if (!osp.isInitial()) {
+						queryToUse = queryToUse.offset(osp.getOffset() + 1);
+					}
 
 					if (limit > 0) {
 						queryToUse = queryToUse.limit(limit + 1);
@@ -419,8 +423,7 @@ public Mono<Window<T>> scroll(ScrollPosition scrollPosition) {
 				}).all() //
 						.collectList() //
 						.map(content -> {
-							return ScrollDelegate.createWindow(content, limit,
-									OffsetScrollPosition.positionFunction(osp.getOffset()));
+							return ScrollDelegate.createWindow(content, limit, osp.positionFunction());
 						});
 			}