[Flutter] Long Press menu widget

call_it_from_implementation.dart

  var _tapPosition;

void _showCustomMenu() {
    var overlay = Overlay.of(context)!.context.findRenderObject() as RenderBox;

    showMenu(
        context: context,
        items: <PopupMenuEntry<int>>[PlusMinusEntry()],
        position: RelativeRect.fromRect(
            _tapPosition & const Size(40, 40), // smaller rect, the touch area
            Offset.zero & overlay.size // Bigger rect, the entire screen
            ));
  }

  void _storePosition(TapDownDetails details) {
    _tapPosition = details.globalPosition;
  }

return GestureDetector(
            // This does not give the tap position ...
            onLongPress: _showCustomMenu,
            // Have to remember it on tap-down.
            onTapDown: _storePosition,
            child: (....)
          );

custom_pop_menu.dart

import 'package:flutter/material.dart' hide showMenu;
import 'package:flutter/material.dart' as material show showMenu;

/// A mixin to provide convenience methods to record a tap position and show a popup menu.
mixin CustomPopupMenu<T extends StatefulWidget> on State<T> {
  late Offset _tapPosition;

  /// Pass this method to an onTapDown parameter to record the tap position.
  void storePosition(TapDownDetails details) => _tapPosition = details.globalPosition;

  /// Use this method to show the menu.
  // ignore: avoid_shadowing_type_parameters
  Future<T?>? showMenu<T>({
    required BuildContext context,
    required List<PopupMenuEntry<T>> items,
    required T initialValue,
    double? elevation,
    String? semanticLabel,
    ShapeBorder? shape,
    Color? color,
    bool captureInheritedThemes = true,
    bool useRootNavigator = false,
  }) {
    var overlay = Overlay.of(context)!.context.findRenderObject() as RenderBox;

    return material.showMenu<T>(
      context: context,
      position: RelativeRect.fromLTRB(
        _tapPosition.dx,
        _tapPosition.dy,
        overlay.size.width - _tapPosition.dx,
        overlay.size.height - _tapPosition.dy,
      ),
      items: items,
      initialValue: initialValue,
      elevation: elevation,
      semanticLabel: semanticLabel,
      shape: shape,
      color: color,
      useRootNavigator: useRootNavigator,
    );
  }
}

class PlusMinusEntry extends PopupMenuEntry<int> {
  @override
  double height = 100;
  // height doesn't matter, as long as we are not giving
  // initialValue to showMenu().
  @override
  PlusMinusEntryState createState() => PlusMinusEntryState();

  @override
  bool represents(int? n) {
    return n == 1 || n == -1;
  }
}

class PlusMinusEntryState extends State<PlusMinusEntry> {
  void _plus1() {
    // This is how you close the popup menu and return user selection.
    Navigator.pop<int>(context, 1);
  }

  void _minus1() {
    Navigator.pop<int>(context, -1);
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      children: <Widget>[
        Expanded(child: TextButton(onPressed: _plus1, child: Text('+1'))),
        Expanded(child: TextButton(onPressed: _minus1, child: Text('-1'))),
      ],
    );
  }
}