scrollable_positioned_list: expose position, offset and add jumpToOffset in ScrollOffsetController

packages/scrollable_positioned_list/lib/src/scrollable_positioned_list.dart

@@ -279,6 +279,10 @@ class ItemScrollController {
 /// This is an experimental API and is subject to change.
 /// Behavior may be ill-defined in some cases.  Please file bugs.
 class ScrollOffsetController {
+
+  ScrollPosition get position => _scrollableListState!.primary.scrollController.position;
+  Offset get offset => _scrollableListState!.primary.scrollController.offset;
+
   Future<void> animateScroll(
       {required double offset,
       required Duration duration,
@@ -293,6 +297,12 @@ class ScrollOffsetController {
     );
   }
 
+  void jumpToOffset(double offset){
+    final currentPosition = _scrollableListState!.primary.scrollController.offset;
+    final newPosition = currentPosition + offset;
+    _scrollableListState!.primary.scrollController.jumpTo(newPosition);
+  }
+
   _ScrollablePositionedListState? _scrollableListState;
 
   void _attach(_ScrollablePositionedListState scrollableListState) {

packages/scrollable_positioned_list/test/scroll_offset_controller_test.dart

@@ -225,4 +225,28 @@ void main() {
 
     await tester.pumpAndSettle();
   });
+
+  testWidgets('Programtically jump down 50 pixels',
+      (WidgetTester tester) async {
+    final scrollDistance = 50.0;
+
+    ScrollOffsetController scrollOffsetController = ScrollOffsetController();
+
+    await setUpWidgetTest(
+      tester,
+      scrollOffsetController: scrollOffsetController,
+      initialIndex: 5,
+    );
+
+    final originalOffest = tester.getTopLeft(find.text('Item 5')).dy;
+
+    scrollOffsetController.jumpToOffset(-scrollDistance);
+    await tester.pumpAndSettle();
+
+    final newOffset = tester.getTopLeft(find.text('Item 5')).dy;
+
+    expect(newOffset - originalOffest, scrollDistance);
+  });
+
+
 }