Test of very tiny join decorations (green is path, blue is 2 lines)

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  StrokeJoin join = StrokeJoin.miter;
  double offset = 1.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            CustomPaint(
              foregroundPainter: _MyPainter(StrokeJoin.miter, offset),
              size: const Size(400, 400),
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceEvenly,
              children: <Widget>[
                DropdownButton(
                  value: join,
                  items: StrokeJoin.values.map((e) {
                    return DropdownMenuItem<StrokeJoin>(
                      value: e,
                      child: Text('$e'),
                    );
                  }).toList(),
                  onChanged: (value) {
                    setState(() {
                      join = value!;
                    });
                  },
                ),
                Slider(
                  value: offset,
                  min: -5.0,
                  max: 5.0,
                  onChanged: (value) {
                    setState(() {
                      offset = value;
                    });
                  },)
              ],
            ),
          ],
        ),
      ),
    );
  }
}

class _MyPainter extends CustomPainter {
  _MyPainter(this.join, this.offset);

  final StrokeJoin join;
  final double offset;

  void drawStringAboveCenter(Canvas canvas, String label, double centerX, double centerY) {
    TextSpan span = TextSpan(text: label, style: const TextStyle(color: Colors.black));
    TextPainter painter = TextPainter(
      text: span,
      textDirection: TextDirection.ltr,
    );
    painter.layout();
    painter.paint(canvas, Offset(centerX - painter.width * 0.5,
                                 centerY - painter.height));
  }

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint();
    paint.style = PaintingStyle.stroke;
    paint.strokeJoin = join;
    paint.strokeWidth = 50.0;

    drawStringAboveCenter(canvas, 'path', 200, 120);
    paint.color = Colors.green;
    {
      Path path = Path();
      path.moveTo(50, 150);
      path.lineTo(150, 150);
      path.lineTo(250, 150 + offset);
      canvas.drawPath(path, paint);
    }
    {
      Path path = Path();
      path.moveTo(350, 50);
      path.lineTo(350, 150);
      path.lineTo(350 + offset, 250);
      canvas.drawPath(path, paint);
    }

    drawStringAboveCenter(canvas, '2 lines', 200, 220);
    paint.color = Colors.blueAccent;
    canvas.drawLine(const Offset(50, 250), const Offset(150, 250), paint);
    canvas.drawLine(const Offset(150, 250), Offset(250, 250 + offset), paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}