Grid gap rendering

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Grid gaps',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Grid Gaps'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: CustomScrollView(
        physics: BouncingScrollPhysics(),
        slivers: <Widget>[
          _buildListHeader('.5 pixel, white'),
          SliverGrid(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 4,
              childAspectRatio: 1,
              mainAxisSpacing: .5,
              crossAxisSpacing: .5,
            ),
            delegate: SliverChildBuilderDelegate(
              _buildChild,
              addRepaintBoundaries: true,
              childCount: 12,
            ),
          ),
          _buildListHeader('1 pixel, white'),
          SliverGrid(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 4,
              childAspectRatio: 1,
              mainAxisSpacing: 1,
              crossAxisSpacing: 1,
            ),
            delegate: SliverChildBuilderDelegate(
              _buildChild,
              addRepaintBoundaries: true,
              childCount: 12,
            ),
          ),
          _buildListHeader('995‰ transform'),
          SliverGrid(
            gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                crossAxisCount: 4, childAspectRatio: 1),
            delegate: SliverChildBuilderDelegate(
              (context, i) => Container(
                color: Colors.black,
                transform: Matrix4.identity()..scale(.995),
              ),
              addRepaintBoundaries: true,
              childCount: 12,
            ),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }

  Widget _buildListHeader(String text) {
    return SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, i) => Container(
          margin: const EdgeInsets.all(16.0),
          child: Text(
            text,
            style: Theme.of(context).textTheme.title,
          ),
        ),
        childCount: 1,
        addAutomaticKeepAlives: false,
      ),
    );
  }

  Widget _buildChild(context, i) {
    return Container(
      color: Colors.black,
    );
  }
}