subtracks/lib/util/custom_scroll_fix.dart

import 'package:flutter/material.dart';

// Fixes scroll postion being reset in a tab when scrolling up inside a
// NestedScrollView.
//
// https://dartpad.dev/?id=5d1a0c7c4b55281ccff377fc8ea90b60
// https://github.com/flutter/flutter/issues/159123#issuecomment-2614343654

class CustomScrollProvider extends StatefulWidget {
  const CustomScrollProvider({
    super.key,
    required this.tabController,
    required this.parent,
    required this.child,
  });

  final TabController tabController;
  final ScrollController parent;
  final Widget child;

  @override
  State<CustomScrollProvider> createState() => _CustomScrollProviderState();
}

class _CustomScrollProviderState extends State<CustomScrollProvider> {
  late final List<CustomScrollController> scrollControllers;

  @override
  void initState() {
    super.initState();

    final activeIndex = widget.tabController.index;

    scrollControllers = List.generate(
      widget.tabController.length,
      (index) => CustomScrollController(
        isActive: index == activeIndex,
        parent: widget.parent,
        debugLabel: 'CustomScrollController/$index',
      ),
    );

    widget.tabController.addListener(() {
      changeActiveIndex(widget.tabController.index);
    });
  }

  @override
  void dispose() {
    for (final scrollController in scrollControllers) {
      scrollController.dispose();
    }

    super.dispose();
  }

  void changeActiveIndex(int value) {
    for (var i = 0; i < scrollControllers.length; i++) {
      final scrollController = scrollControllers[i];
      final isActive = i == value;
      scrollController.isActive = isActive;

      if (isActive) {
        scrollController.forceAttach();
      } else {
        scrollController.forceDetach();
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    return CustomScrollProviderData(
      scrollControllers: scrollControllers,
      child: widget.child,
    );
  }
}

class CustomScrollProviderData extends InheritedWidget {
  const CustomScrollProviderData({
    super.key,
    required super.child,
    required this.scrollControllers,
  });

  static CustomScrollProviderData of(BuildContext context) {
    return context
        .dependOnInheritedWidgetOfExactType<CustomScrollProviderData>()!;
  }

  final List<CustomScrollController> scrollControllers;

  @override
  bool updateShouldNotify(CustomScrollProviderData oldWidget) {
    return scrollControllers != oldWidget.scrollControllers;
  }
}

class CustomScrollController extends ScrollController {
  CustomScrollController({
    required this.isActive,
    required this.parent,
    String debugLabel = 'CustomScrollController',
  }) : super(
         debugLabel: parent.debugLabel == null
             ? null
             : '${parent.debugLabel}/$debugLabel',
         initialScrollOffset: parent.initialScrollOffset,
         keepScrollOffset: parent.keepScrollOffset,
       );

  bool isActive;
  final ScrollController parent;

  @override
  ScrollPosition createScrollPosition(
    ScrollPhysics physics,
    ScrollContext context,
    ScrollPosition? oldPosition,
  ) {
    debugPrint('$debugLabel-createScrollPosition: $isActive');

    return parent.createScrollPosition(physics, context, oldPosition);
  }

  @override
  void attach(ScrollPosition position) {
    debugPrint('$debugLabel-attach: $isActive');

    super.attach(position);
    if (isActive && !parent.positions.contains(position)) {
      parent.attach(position);
    }
  }

  @override
  void detach(ScrollPosition position) {
    debugPrint('$debugLabel-detach: $isActive');

    if (parent.positions.contains(position)) {
      parent.detach(position);
    }

    super.detach(position);
  }

  void forceDetach() {
    debugPrint('$debugLabel-forceDetach: $isActive');

    for (final position in positions) {
      if (parent.positions.contains(position)) {
        parent.detach(position);
      }
    }
  }

  void forceAttach() {
    debugPrint('$debugLabel-forceAttach: $isActive');

    for (final position in positions) {
      if (!parent.positions.contains(position)) {
        parent.attach(position);
      }
    }
  }

  @override
  void dispose() {
    debugPrint('$debugLabel-dispose: $isActive');

    forceDetach();
    super.dispose();
  }
}