anchor.diff:      bool addRepaintBoundaries = true,
       bool addAutomaticKeepAlives = true,
       double minCacheExtent,
  +    bool variableHeight = false,
     }) async {
       tester.binding.window.devicePixelRatioTestValue = 1.0;
       tester.binding.window.physicalSizeTestValue =
  
positive.code:             .itemLeadingEdge,
          0.5);
    });
  
    testWidgets('Scroll to 100 and position at bottom',
        (WidgetTester tester) async {
  
negative.code:           itemBuilder: (context, index) {
              assert(index >= 0 && index <= itemCount - 1);
              return SizedBox(
                height:
                    variableHeight ? (itemHeight + (index % 13) * 5) : itemHeight,
                child: Text('Item $index'),
              );
            },
  