Disappearing Box

main.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'The magically disappearing box...'),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Stack(
        children: [
          ListView.builder(
            itemCount: 1000,
            itemBuilder: (context, idx) {
              return ListTile(
                title: Text(idx.toString()),
              );
            },
          ),
          Center(
            child: SizedBox(
              width: 256,
              height: 256,
              child: FlatButton(
                child: Houdini(),
                onPressed: () {
                  //intentionally blank, showing how item appears on hover
                },
              ),
            ),
          ),
        ],
      ),
    );
  }
}

// A simple LeafRenderObjectWidget that draws a circle.
class Houdini extends LeafRenderObjectWidget {
  const Houdini();

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _HoudiniRenderObject();
  }

  @override
  void updateRenderObject(
      BuildContext context, covariant _HoudiniRenderObject renderObject) {
    ;
  }

  @override
  void didUnmountRenderObject(covariant _HoudiniRenderObject renderObject) {}
}

class _HoudiniRenderObject extends RenderBox {
  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    markNeedsPaint();
  }

  @override
  bool hitTestSelf(Offset screenOffset) => false;

  @override
  void performResize() {
    size = constraints.biggest;
  }

  @override
  bool get sizedByParent => true;

  @override
  void paint(PaintingContext context, Offset offset) {
    var canvas = context.canvas;

    canvas.save();

    bool showDisappearBug = true;
    if (showDisappearBug) {
      // This path shows the bug
      var rect = Rect.fromLTWH(100, 100, 512, 512);
      var scale = 0.25;
      canvas.translate(offset.dx, offset.dy);
      canvas.translate(size.width / 2 - rect.width * scale / 2,
          size.height / 2 - rect.height * scale / 2);
      canvas.scale(scale);
      canvas.translate(-rect.left, -rect.top);

      canvas.drawRect(
        rect,
        Paint()
          ..color = Colors.red
          ..blendMode = BlendMode.srcIn,
      );
      canvas.restore();
    } else {
      // This version with no custom transform works.
      var rect = offset & size;

      canvas.drawRect(
        rect,
        Paint()
          ..color = Colors.red
          ..blendMode = BlendMode.srcIn,
      );
      canvas.restore();
    }
  }
}