TightClipper to avoid some AA artifacts

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(IssueDemoApp());
}

class IssueDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.indigo,
        scaffoldBackgroundColor: Colors.grey[100],
        buttonTheme: ButtonThemeData(
          colorScheme: ColorScheme.fromSwatch(primarySwatch: Colors.indigo),
          textTheme: ButtonTextTheme.primary,
        ),
      ),
      debugShowCheckedModeBanner: false,
      home: ClipRectIssueDemo(),
    );
  }
}

class ClipRectIssueDemo extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _ClipRectIssueDemoState();
  }
}

class _ClipRectIssueDemoState extends State<ClipRectIssueDemo> {
  bool _clipRectOn = false;
  double _width = 219.0;

  @override
  Widget build(BuildContext context) {
    final double pixelRatio = MediaQuery.of(context).devicePixelRatio;
    return Scaffold(
      appBar: AppBar(
        title: const Text('ClipRect Issue Demo'),
        centerTitle: true,
        elevation: 0,
      ),
      body: SingleChildScrollView(
        child: Column(
          children: [
            // And then some gap space too
            const SizedBox(height: 20),
            Text('ClipRect over Container with BoxShadow (pixel ratio: $pixelRatio)',
                style: Theme.of(context).textTheme.headline6),
            const SizedBox(height: 20),

            SizedBox(
              width: 420,
              height: 420,
              child: Center(
                child: SizedBox(
                  height: _width,
                  width: _width,
                  child: ClipRectDecoratedContainer(clipRectOn: _clipRectOn),
                ),
              ),
            ),

            const SizedBox(height: 10),
            Center(
              child: SizedBox(
                width: 450,
                child: SwitchListTile(
                  title: const Text('ClipRect ON/OFF'),
                  subtitle: const Text(
                      'Turn on ClipRect to see edge remnants. \nIf you resize '
                      'window/media size or change container size, you can observe the edge '
                      'remnants appearing and dissapearing at different sizes.'),
                  value: _clipRectOn,
                  onChanged: (value) {
                    setState(() {
                      _clipRectOn = value;
                    });
                  },
                ),
              ),
            ),

            const SizedBox(height: 10),

            Center(
              child: SizedBox(
                width: 450,
                child: ListTile(
                  title: const Text('Change Container size'),
                  subtitle: Slider(
                    min: 100.0,
                    max: 400.0,
                    divisions: (400 - 100).floor(),
                    label: _width.floor().toString(),
                    value: _width,
                    onChanged: (value) {
                      setState(() {
                        _width = value;
                      });
                    },
                  ),
                  trailing: Padding(
                    padding: const EdgeInsets.only(right: 12.0),
                    child: Column(
                      crossAxisAlignment: CrossAxisAlignment.end,
                      children: <Widget>[
                        const Text(
                          'Width',
                          style: TextStyle(fontSize: 11),
                        ),
                        Text(
                          _width.floor().toString(),
                          style: const TextStyle(fontSize: 15),
                        ),
                      ],
                    ),
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class ClipRectDecoratedContainer extends StatelessWidget {
  const ClipRectDecoratedContainer({
    Key key,
    this.clipRectOn = false,
  }) : super(key: key);

  final bool clipRectOn;

  @override
  Widget build(BuildContext context) {
    if (clipRectOn) {
      return ClipRect(
        clipBehavior: Clip.hardEdge,
        clipper: TightClipper(MediaQuery.of(context).devicePixelRatio),
        child: Container(
          decoration: BoxDecoration(
            color: Colors.white,
            boxShadow: [
              BoxShadow(
                color: Colors.red[900],
                blurRadius: 25.0,
              )
            ],
          ),
        ),
      );
    } else {
      return Container(
        decoration: BoxDecoration(
          color: Colors.white,
          boxShadow: [
            BoxShadow(
              color: Colors.red[900],
              blurRadius: 25.0,
            )
          ],
        ),
      );
    }
  }
}

class TightClipper extends CustomClipper<Rect> {
  TightClipper(this.devicePixelRatio, {this.tightFactor = 1});

  @override
  Rect getClip(Size size) {
    final double padding = 1 / devicePixelRatio * tightFactor;
    return Rect.fromLTRB(
      padding,
      padding,
      size.width - padding,
      size.height - padding,
    );
  }

  @override
  bool shouldReclip(CustomClipper<Rect> oldClipper) {
    return true;
  }

  final double devicePixelRatio;
  final double tightFactor;
}

In line 189: I would expect that you only need to reclip if the provided oldClipper has a different runtime type or if devicePixelRatio / tightFactor have changed.

Ah yes. I'll revise this quick hack if we're going to put this piece of code in our doc, or code base.