It doesn't work as shown in the ReadMe.

[darkstarx]

If you don't set a ScrollController to the widget, you'll get the assertion:

dependOnInheritedWidgetOfExactType<PrimaryScrollController>() or dependOnInheritedElement() was called before _ScrollShadowState.initState() completed.

import 'package:flutter/material.dart';
import 'package:flutter_scroll_shadow/flutter_scroll_shadow.dart';


void main()
{
  runApp(const MyApp());
}


class MyApp extends StatelessWidget
{
  const MyApp({ super.key });

  @override
  Widget build(final BuildContext context)
  {
    return MaterialApp(
      theme: ThemeData.light(),
      darkTheme: ThemeData.dark(),
      themeMode: ThemeMode.dark,
      routes: {
        '/page1': (context) => const Page1(),
      },
      initialRoute: '/page1',
    );
  }
}


class Page1 extends StatefulWidget
{
  const Page1({ super.key });

  @override
  State<Page1> createState() => _Page1State();
}


class _Page1State extends State<Page1>
{
  @override
  Widget build(final BuildContext context)
  {
    return Scaffold(
      appBar: AppBar(title: const Text('Page1')),
      body: const MyScrollView(),
    );
  }
}


class MyScrollView extends StatelessWidget
{
  const MyScrollView({ super.key });

  @override
  Widget build(final BuildContext context)
  {
    return ScrollShadow(
      color: Colors.grey,
      child: SingleChildScrollView(
        child: Column(
          children: List
            .generate(20, (i) => SizedBox(height: 100, child: Text('Item #$i')))
            .toList()
        ),
      ),
    );
  }
}

[darkstarx]

You can't use PrimaryScrollController.of(context) or any other dependOnInheritedWidgetOfExactType until the initState is completed.