To show popupmenu without using PopupMenuButton since it only works with onTap.

main.dart

  showCustomMenu(context) {
    final RenderBox overlay = Overlay.of(context).context.findRenderObject();

    showMenu(
      context: context,
      position: RelativeRect.fromRect(
          _tapPosition & Size(40, 40), 
          Offset.zero & overlay.size 
          ),
      items: [
        PopupMenuItem(
          child: Text('Copy'),
        ),
        PopupMenuItem(
          child: Text('Reply'),
        ),
      ],
    );
  }