[scrollable_positioned_list] expose ScrollPosition as unstableScrollPosition in ScrollOffsetController

packages/scrollable_positioned_list/lib/src/scrollable_positioned_list.dart

@@ -279,6 +279,15 @@ 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 of the current ScrollablePositionedList. Values such as
+  /// pixels, maxScrollExtent and jumpTo are not necessarily defined to start
+  /// from the beginning of the list. When itemScrollController.jumpTo is
+  /// called, the list will begin from that index. unstableScrollPosition.jumpTo
+  /// will cause the application to freeze at very large values as it must build
+  /// all the widgets between the starting offset and the ending offset.
+  ScrollPosition get unstableScrollPosition =>
+      _scrollableListState!.primary.scrollController.position;
+
   Future<void> animateScroll(
       {required double offset,
       required Duration duration,

packages/scrollable_positioned_list/test/scroll_offset_controller_test.dart

@@ -225,4 +225,30 @@ 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 originalOffset = tester.getTopLeft(find.text('Item 5')).dy;
+
+    final currentPosition =
+        scrollOffsetController.unstableScrollPosition.pixels;
+    final newPosition = currentPosition - scrollDistance;
+
+    scrollOffsetController.unstableScrollPosition.jumpTo(newPosition);
+    await tester.pumpAndSettle();
+
+    final newOffset = tester.getTopLeft(find.text('Item 5')).dy;
+
+    expect(newOffset - originalOffset, scrollDistance);
+  });
 }