Mail doesn't open on Flutter Web when body content is too long

main.dart

import 'package:flutter/cupertino.dart';
import 'package:mailto/mailto.dart';
import 'package:url_launcher/url_launcher.dart';

void main() => runApp(MailtoExampleApp());

class MailtoExampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      debugShowCheckedModeBanner: false,
      theme: CupertinoThemeData(
        textTheme: CupertinoTextThemeData(
          navLargeTitleTextStyle: TextStyle(
            fontWeight: FontWeight.bold,
            fontSize: 22.0,
            color: CupertinoColors.activeGreen,
          ),
        ),
      ),
      home: MailtoDemo(),
    );
  }
}

const Color white = Color(0xFFFFFFFF);
const EdgeInsets textFieldPadding = EdgeInsets.all(16);
const EdgeInsets iconPadding = EdgeInsets.fromLTRB(12, 0, 12, 0);

class MailtoDemo extends StatefulWidget {
  @override
  _MailtoDemoState createState() => _MailtoDemoState();
}

class _MailtoDemoState extends State<MailtoDemo> {
  List<String> to = [];
  List<String> cc = [];
  List<String> bcc = [];
  String subject = '';
  String body = '';

  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      navigationBar: CupertinoNavigationBar(
        backgroundColor: white,
        middle: Text(
          'mailto',
        ),
      ),
      child: ListView(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.symmetric(
              horizontal: 12,
            ),
            child: CupertinoButton(
              color: Color(0xFF8E44AD),
              child: Text('Suprise me!'),
              onPressed: () async {
                final url = Mailto(
                  to: [
                    'example@example.com',
                    'ejemplo@ejemplo.com',
                  ],
                  cc: [
                    'percentage%100@example.com',
                    'QuestionMark?address@example.com',
                  ],
                  bcc: [
                    'Mike&family@example.org',
                  ],
                  subject: 'Let\'s drink a "café"! ☕️ 2+2=4 #coffeeAndMath',
                  body:
                      "There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don\'t look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc. There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc.",
                ).toString();
                if (await canLaunch(url)) {
                  await launch(url);
                } else {
                  showCupertinoDialog(
                    context: context,
                    builder: MailClientOpenErrorDialog(url: url).build,
                  );
                }
              },
            ),
          ),
          EmailsContainer(
            onChanged: (v) => to = v,
            icon: CupertinoIcons.person_solid,
            title: 'to',
            placeholder: 'Recipient',
          ),
          EmailsContainer(
            onChanged: (v) => cc = v,
            icon: CupertinoIcons.group_solid,
            title: 'cc',
            placeholder: 'Carbon-copy',
          ),
          EmailsContainer(
            onChanged: (v) => bcc = v,
            icon: CupertinoIcons.group,
            title: 'bcc',
            placeholder: 'Blind Carbon-copy',
          ),
          SubjectTextField(
            onChanged: (v) => subject = v,
          ),
          BodyTextField(
            onChanged: (v) => body = v,
          ),
          Padding(
            padding: const EdgeInsets.symmetric(
              vertical: 32,
              horizontal: 12,
            ),
            child: CupertinoButton.filled(
              child: Text('Open Mail Client'),
              onPressed: () async {
                final url = Mailto(
                  to: to,
                  cc: cc,
                  bcc: bcc,
                  subject: subject,
                  body: body,
                ).toString();
                if (await canLaunch(url)) {
                  await launch(url);
                } else {
                  showCupertinoDialog(
                    context: context,
                    builder: MailClientOpenErrorDialog(url: url).build,
                  );
                }
              },
            ),
          ),
        ],
      ),
    );
  }
}

class MailClientOpenErrorDialog extends StatelessWidget {
  final String url;

  const MailClientOpenErrorDialog({Key key, @required this.url})
      : assert(url != null),
        assert(url != ''),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return CupertinoAlertDialog(
      title: Text('Launch Error'),
      content: Text('We could not launch the following url:\n$url'),
      actions: <Widget>[
        CupertinoDialogAction(
          isDefaultAction: true,
          child: Text('OK'),
          onPressed: () {
            Navigator.of(context).pop();
          },
        ),
      ],
    );
  }
}

class BodyTextField extends StatefulWidget {
  final ValueChanged<String> onChanged;

  const BodyTextField({
    Key key,
    @required this.onChanged,
  })  : assert(onChanged != null),
        super(key: key);

  @override
  _BodyTextFieldState createState() => _BodyTextFieldState();
}

class _BodyTextFieldState extends State<BodyTextField> {
  final TextEditingController _controller = TextEditingController();

  bool isEnabled = false;

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        SectionHeading(
          leadingIcon: CupertinoIcons.book,
          title: 'body',
          trailingIcon:
              isEnabled ? CupertinoIcons.delete : CupertinoIcons.add_circled,
          onPressed: () {
            setState(() => isEnabled = !isEnabled);
            _controller.text = '';
            if (isEnabled) {
              widget.onChanged(_controller.text);
            } else {
              widget.onChanged(null);
            }
          },
        ),
        if (isEnabled)
          Padding(
            padding: const EdgeInsets.fromLTRB(12, 4, 12, 4),
            child: CupertinoTextField(
              controller: _controller,
              padding: textFieldPadding,
              minLines: 4,
              maxLines: null,
              textCapitalization: TextCapitalization.sentences,
              keyboardType: TextInputType.multiline,
              placeholder: 'Body of the email',
              onChanged: widget.onChanged,
            ),
          ),
      ],
    );
  }
}

class SectionHeading extends StatelessWidget {
  final String title;
  final VoidCallback onPressed;
  final IconData leadingIcon;
  final IconData trailingIcon;

  const SectionHeading({
    Key key,
    @required this.title,
    @required this.onPressed,
    @required this.leadingIcon,
    @required this.trailingIcon,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Flexible(
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: <Widget>[
          Row(
            children: <Widget>[
              Padding(
                padding: iconPadding,
                child: Icon(leadingIcon),
              ),
              LargeText(title),
            ],
          ),
          Padding(
            padding: const EdgeInsets.only(right: 4),
            child: CupertinoButton(
              child: Icon(trailingIcon),
              onPressed: onPressed,
            ),
          ),
        ],
      ),
    );
  }
}

class SubjectTextField extends StatefulWidget {
  final ValueChanged<String> onChanged;

  const SubjectTextField({
    Key key,
    @required this.onChanged,
  })  : assert(onChanged != null),
        super(key: key);

  @override
  _SubjectTextFieldState createState() => _SubjectTextFieldState();
}

class _SubjectTextFieldState extends State<SubjectTextField> {
  final TextEditingController _controller = TextEditingController();

  bool isEnabled = false;

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        SectionHeading(
          leadingIcon: CupertinoIcons.mail,
          title: 'subject',
          trailingIcon:
              isEnabled ? CupertinoIcons.delete : CupertinoIcons.add_circled,
          onPressed: () {
            setState(() => isEnabled = !isEnabled);
            _controller.text = '';
            if (isEnabled) {
              widget.onChanged(_controller.text);
            } else {
              widget.onChanged(null);
            }
          },
        ),
        if (isEnabled)
          Padding(
            padding: const EdgeInsets.fromLTRB(12, 4, 12, 4),
            child: CupertinoTextField(
              controller: _controller,
              padding: textFieldPadding,
              textCapitalization: TextCapitalization.words,
              placeholder: 'Subject of the email',
              onChanged: widget.onChanged,
            ),
          ),
      ],
    );
  }
}

class LargeText extends StatelessWidget {
  final String data;

  const LargeText(this.data, {Key key})
      : assert(data != null),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(vertical: 16),
      child: Text(
        data,
        style: CupertinoTheme.of(context)
            .textTheme
            .navLargeTitleTextStyle
            .copyWith(color: CupertinoColors.black),
      ),
    );
  }
}

class EmailsContainer extends StatefulWidget {
  const EmailsContainer({
    Key key,
    @required this.title,
    @required this.icon,
    @required this.onChanged,
    @required this.placeholder,
  }) : super(key: key);

  final String title;
  final String placeholder;
  final IconData icon;
  final ValueChanged<List<String>> onChanged;

  @override
  _EmailsContainerState createState() => _EmailsContainerState();
}

class _EmailsContainerState extends State<EmailsContainer> {
  final List<TextEditingController> _controllers = [];

  @override
  void dispose() {
    _controllers.forEach((c) => c.dispose());
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        SectionHeading(
          leadingIcon: widget.icon,
          trailingIcon: CupertinoIcons.plus_circled,
          title: widget.title,
          onPressed: () {
            setState(() {
              _controllers.add(TextEditingController(text: ''));
            });
            callOnChanged();
          },
        ),
        ...buildTextFields(),
      ],
    );
  }

  void callOnChanged() {
    widget.onChanged(_controllers.map((c) => c.text.trim()).toList());
  }

  void removeController(int index) {
    assert(index >= 0);
    assert(index < _controllers.length);
    setState(() => _controllers.removeAt(index));
    callOnChanged();
  }

  List<Widget> buildTextFields() {
    final widgets = <Widget>[];
    _controllers.asMap().forEach((index, controller) {
      widgets.add(Padding(
        padding: const EdgeInsets.fromLTRB(12, 4, 12, 4),
        child: CupertinoTextField(
          keyboardType: TextInputType.emailAddress,
          controller: controller,
          padding: textFieldPadding,
          suffix: CupertinoButton(
            padding: EdgeInsets.zero,
            onPressed: () => removeController(index),
            child: Padding(
              padding: const EdgeInsets.fromLTRB(16, 8, 8, 8),
              child: Icon(CupertinoIcons.minus_circled),
            ),
          ),
          textCapitalization: TextCapitalization.none,
          placeholder: '${widget.placeholder} ${index + 1}',
          onChanged: (_) => callOnChanged(),
        ),
      ));
    });
    return widgets;
  }
}