Flutter scrollable footer - a footer that stays at the bottom of the screen and scrolls with its container; also see https://docs.flutter.dev/flutter/widgets/SingleChildScrollView-class.html

scrollable_footer.dart

import 'package:flutter/material.dart';

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

class ScrollableFooter extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
        builder: (BuildContext context, BoxConstraints constraints) {
      return SingleChildScrollView(
        child: ConstrainedBox(
          constraints: constraints.copyWith(
            minHeight: constraints.maxHeight,
            maxHeight: double.infinity,
          ),
          child: IntrinsicHeight(
            child: Column(
              children: <Widget>[
                Container(height: 200, color: Colors.blue),
                Container(height: 200, color: Colors.orange),
                Container(height: 200, color: Colors.green),
                Container(height: 50, color: Colors.pink),
                Expanded(
                  child: Align(
                    alignment: Alignment.bottomCenter,
                    child: Container(
                      width: double.infinity,
                      color: Colors.red,
                      padding: EdgeInsets.all(12.0),
                      child: Text(
                        'FOOTER',
                        textAlign: TextAlign.center,
                      ),
                    ),
                  ),
                ),
              ],
            ),
          ),
        ),
      );
    });
  }
}