Flutter's GIF renderer is too compatible with the GIF89a spec unlike the rest of the world

main.dart

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

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

class MyApp extends StatefulWidget {
  const MyApp({super.key});
  
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  // Delay in milliseconds. Valid range is 0-655350.
  int blackDelayMs = 500;
  int redDelayMs = 0;
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Column(
            children: [
              Slider(
                min: 0,
                max: 1000,
                divisions: 100,
                activeColor: Colors.black,
                value: blackDelayMs.toDouble(),
                label: "${blackDelayMs.round()} ms",
                onChanged: (double value) {
                  setState(() {
                    blackDelayMs = value.round().toInt();
                  });
                },
              ),
              Slider(
                 min: 0,
                 max: 1000,
                 divisions: 100,
                 activeColor: Colors.red,
                 value: redDelayMs.toDouble(),
                 label: "${redDelayMs.round()} ms",
                 onChanged: (double value) {
                   setState(() {
                     redDelayMs = value.round().toInt();
                   });
                 },
               ),
              SizedBox(
                width: 200,
                height: 200,
                child: FittedBox(
                  fit: BoxFit.contain,
                  child: Image.memory(generateGif(
                    blackDelayMs: blackDelayMs,
                    redDelayMs: redDelayMs,
                  )),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

// GIF data follows.

List<int> toBytes(int millis) {
  final hundreds = millis ~/ 10;
  return [hundreds & 0xff, (hundreds & 0xffff) >>> 8];
}

Uint8List generateGif({required int blackDelayMs, required int redDelayMs}) {
  return Uint8List.fromList([
    0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x50, 0x00,
    0x50, 0x00, 0xf1, 0x00, 0x00, 0x00, 0x00, 0x00,
    0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0xff,
    0x00, 0x21, 0xff, 0x0b, 0x4e, 0x45, 0x54, 0x53,
    0x43, 0x41, 0x50, 0x45, 0x32, 0x2e, 0x30, 0x03,
    0x01, 0x00, 0x00, 0x00, 0x21, 0xf9, 0x04, 0x04,
    ...toBytes(blackDelayMs), 0x00, 0x00, 0x2c, 0x00, 0x00, 0x00,
    0x00, 0x50, 0x00, 0x50, 0x00, 0x00, 0x02, 0xb6,
    0x8c, 0x8f, 0xa9, 0xcb, 0x08, 0x0f, 0xa3, 0x9c,
    0xb4, 0xc6, 0x86, 0x33, 0xb6, 0xbc, 0x73, 0x0d,
    0x86, 0x81, 0x47, 0x96, 0x80, 0x88, 0x36, 0xe6,
    0x6a, 0xa5, 0x6e, 0xc2, 0xc6, 0xd2, 0x4b, 0x8f,
    0xf2, 0x5d, 0xbf, 0x37, 0x9e, 0xa7, 0xbb, 0xdc,
    0xf3, 0xfd, 0x58, 0x41, 0xd4, 0x90, 0x58, 0x0c,
    0x1d, 0x57, 0x49, 0xe5, 0xb2, 0xd4, 0x04, 0x3d,
    0xa1, 0xd1, 0xcc, 0x94, 0x54, 0xb5, 0x5e, 0x3b,
    0xd9, 0xcd, 0xf6, 0xd3, 0x65, 0x7c, 0xc1, 0x61,
    0xc5, 0xb8, 0x55, 0x36, 0x9f, 0x29, 0x69, 0xf5,
    0x7a, 0xd6, 0x76, 0xbc, 0xe1, 0x71, 0xc3, 0x9c,
    0x5e, 0xbf, 0x5f, 0xea, 0x76, 0xfd, 0x83, 0xdf,
    0xe7, 0x07, 0x68, 0x23, 0x08, 0xe8, 0xf7, 0x67,
    0x78, 0x38, 0x78, 0x78, 0x92, 0x58, 0xc8, 0xc7,
    0xb8, 0xa8, 0xe8, 0xa8, 0x27, 0xf9, 0x98, 0x37,
    0x09, 0x99, 0x89, 0x79, 0x19, 0x17, 0x49, 0x79,
    0x67, 0xc9, 0x38, 0x4a, 0x5a, 0x6a, 0x7a, 0x8a,
    0x9a, 0xaa, 0xba, 0xca, 0xda, 0xea, 0xfa, 0x0a,
    0x1b, 0x2b, 0x3b, 0x4b, 0x5b, 0x6b, 0x7b, 0x8b,
    0x9b, 0xab, 0xbb, 0xcb, 0xdb, 0xeb, 0xfb, 0x0b,
    0x1c, 0x2c, 0x3c, 0x4c, 0x5c, 0x6c, 0x7c, 0x8c,
    0x9c, 0xac, 0xbc, 0xcc, 0xdc, 0xec, 0xfc, 0x0c,
    0x1d, 0x2d, 0x3d, 0xed, 0x51, 0x00, 0x00, 0x21,
    0xf9, 0x04, 0x04, ...toBytes(redDelayMs), 0x00, 0x00, 0x2c,
    0x28, 0x00, 0x00, 0x00, 0x28, 0x00, 0x28, 0x00,
    0x00, 0x02, 0x27, 0x94, 0x8f, 0xa9, 0xcb, 0xed,
    0x0f, 0xa3, 0x9c, 0xb4, 0xda, 0x8b, 0xb3, 0xde,
    0xbc, 0xfb, 0x0f, 0x86, 0xe2, 0x48, 0x96, 0xe6,
    0x89, 0xa6, 0xea, 0xca, 0xb6, 0xee, 0x0b, 0xc7,
    0xf2, 0x4c, 0xd7, 0xf6, 0x8d, 0xe7, 0xfa, 0xce,
    0x23, 0x05, 0x00, 0x21, 0xf9, 0x04, 0x04, ...toBytes(blackDelayMs),
          0x00, 0x00, 0x2c, 0x28, 0x00, 0x00, 0x00,
    0x28, 0x00, 0x28, 0x00, 0x00, 0x02, 0x27, 0x8c,
    0x8f, 0xa9, 0xcb, 0xed, 0x0f, 0xa3, 0x9c, 0xb4,
    0xda, 0x8b, 0xb3, 0xde, 0xbc, 0xfb, 0x0f, 0x86,
    0xe2, 0x48, 0x96, 0xe6, 0x89, 0xa6, 0xea, 0xca,
    0xb6, 0xee, 0x0b, 0xc7, 0xf2, 0x4c, 0xd7, 0xf6,
    0x8d, 0xe7, 0xfa, 0xce, 0x23, 0x05, 0x00, 0x21,
    0xf9, 0x04, 0x04, ...toBytes(redDelayMs), 0x00, 0x00, 0x2c,
    0x28, 0x00, 0x28, 0x00, 0x28, 0x00, 0x28, 0x00,
    0x00, 0x02, 0x27, 0x94, 0x8f, 0xa9, 0xcb, 0xed,
    0x0f, 0xa3, 0x9c, 0xb4, 0xda, 0x8b, 0xb3, 0xde,
    0xbc, 0xfb, 0x0f, 0x86, 0xe2, 0x48, 0x96, 0xe6,
    0x89, 0xa6, 0xea, 0xca, 0xb6, 0xee, 0x0b, 0xc7,
    0xf2, 0x4c, 0xd7, 0xf6, 0x8d, 0xe7, 0xfa, 0xce,
    0x23, 0x05, 0x00, 0x21, 0xf9, 0x04, 0x04, ...toBytes(blackDelayMs),
          0x00, 0x00, 0x2c, 0x28, 0x00, 0x28, 0x00,
    0x28, 0x00, 0x28, 0x00, 0x00, 0x02, 0x27, 0x8c,
    0x8f, 0xa9, 0xcb, 0xed, 0x0f, 0xa3, 0x9c, 0xb4,
    0xda, 0x8b, 0xb3, 0xde, 0xbc, 0xfb, 0x0f, 0x86,
    0xe2, 0x48, 0x96, 0xe6, 0x89, 0xa6, 0xea, 0xca,
    0xb6, 0xee, 0x0b, 0xc7, 0xf2, 0x4c, 0xd7, 0xf6,
    0x8d, 0xe7, 0xfa, 0xce, 0x23, 0x05, 0x00, 0x21,
    0xf9, 0x04, 0x04, ...toBytes(redDelayMs), 0x00, 0x00, 0x2c,
    0x00, 0x00, 0x28, 0x00, 0x28, 0x00, 0x28, 0x00,
    0x00, 0x02, 0x27, 0x94, 0x8f, 0xa9, 0xcb, 0xed,
    0x0f, 0xa3, 0x9c, 0xb4, 0xda, 0x8b, 0xb3, 0xde,
    0xbc, 0xfb, 0x0f, 0x86, 0xe2, 0x48, 0x96, 0xe6,
    0x89, 0xa6, 0xea, 0xca, 0xb6, 0xee, 0x0b, 0xc7,
    0xf2, 0x4c, 0xd7, 0xf6, 0x8d, 0xe7, 0xfa, 0xce,
    0x23, 0x05, 0x00, 0x21, 0xf9, 0x04, 0x04, ...toBytes(blackDelayMs),
          0x00, 0x00, 0x2c, 0x00, 0x00, 0x28, 0x00,
    0x28, 0x00, 0x28, 0x00, 0x00, 0x02, 0x27, 0x8c,
    0x8f, 0xa9, 0xcb, 0xed, 0x0f, 0xa3, 0x9c, 0xb4,
    0xda, 0x8b, 0xb3, 0xde, 0xbc, 0xfb, 0x0f, 0x86,
    0xe2, 0x48, 0x96, 0xe6, 0x89, 0xa6, 0xea, 0xca,
    0xb6, 0xee, 0x0b, 0xc7, 0xf2, 0x4c, 0xd7, 0xf6,
    0x8d, 0xe7, 0xfa, 0xce, 0x23, 0x05, 0x00, 0x3b
  ]);
}