OverflowFallback

main.dart

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

typedef OverflowCallback = void Function(double minimalHeight);

class RenderOverflowFallback extends RenderMetaData {
  bool shouldClip = false;

  @override
  void performLayout() {
    super.performLayout();
    final minimumPossibleHeight = child?.getMinIntrinsicHeight(size.width) ?? 0;

    final overflow = minimumPossibleHeight - size.height;

    if (overflow > 0) {
      final constraints =
          this.constraints.copyWith(maxHeight: minimumPossibleHeight);
      this.child.layout(constraints);

      OverflowCallback callback = metaData;
      callback?.call(minimumPossibleHeight);

      shouldClip = true;
    }
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (shouldClip) {
      final rect = Offset.zero & size;
      context.pushClipRect(needsCompositing, offset, rect, super.paint,
          clipBehavior: Clip.hardEdge);
    } else {
      super.paint(context, offset);
    }
  }
}

class OverflowFallback extends MetaData {
  final OverflowCallback onOverflow;
  OverflowFallback({this.onOverflow, Widget child})
      : super(child: child, metaData: onOverflow);

  @override
  RenderMetaData createRenderObject(BuildContext context) {
    return RenderOverflowFallback()..metaData = metaData;
  }
}

class AutoScrollable extends StatefulWidget {
  final Widget child;

  const AutoScrollable({Key key, this.child}) : super(key: key);

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

class _AutoScrollableState extends State<AutoScrollable> {
  KeyedSubtree _child;
  double height;

  @override
  void initState() {
    _child = KeyedSubtree(child: widget.child, key: GlobalKey());
    super.initState();
  }

  @override
  void reassemble() {
    if (height != null) {
      setState(() {
        // not clear to me in which cases we need reset this
        // possibly we need to continously check minimumPossibleHeight
        height = null;
      });
    }
    super.reassemble();
  }

  @override
  Widget build(BuildContext context) {
    if (height != null) {
      return SingleChildScrollView(
        child: SizedBox(
          height: height,
          child: _child,
        ),
      );
    } else {
      return OverflowFallback(
        child: _child,
        onOverflow: (height) {
          Future.microtask(() {
            setState(() {
              this.height = height;
            });
          });
        },
      );
    }
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(),
        body: Container(
          color: Colors.yellow,
          padding: EdgeInsets.all(10),
          child: AutoScrollable(
            child: Column(
              children: <Widget>[
                Container(
                  color: Colors.red,
                  height: 100,
                ),
                Text('Hello world'),
                Container(
                  color: Colors.blue,
                  height: 2000,
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}