Issue 1:
Steps to Reproduce

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Center(
child: Stack(
overflow: Overflow.visible,
children: <Widget>[
Container(
color: Colors.green,
constraints: BoxConstraints.tightFor(width: 50.0, height: 50.0),
),
Positioned(
top: 25.0,
left: 25.0,
child: GestureDetector(
behavior: HitTestBehavior.opaque,
onTap: () {
print('onTap ${DateTime.now()}');
},
child: Container(
color: Colors.blue,
constraints:
BoxConstraints.tightFor(width: 50.0, height: 50.0),
)),
),
],
),
));
}
}

Tap the part of the blue rectangle that is inside the green one: onTap works

Tap the blue rectangle that is outside of the green one: onTap doesn't work

[✓] Flutter (Channel dev, v0.5.7, on Mac OS X 10.14 18A353d, locale en-EE)
• Flutter version 0.5.7 at /Users/maidokaara/Programs/flutter
• Framework revision 66091f9696 (5 weeks ago), 2018-07-09 12:52:41 -0700
• Engine revision 6fe748490d
• Dart version 2.0.0-dev.63.0.flutter-4c9689c1d2

[✓] Android toolchain - develop for Android devices (Android SDK 28.0.2)
• Android SDK at /Users/maidokaara/Library/Android/sdk
• Android NDK at /Users/maidokaara/Library/Android/sdk/ndk-bundle
• Platform android-28, build-tools 28.0.2
• ANDROID_HOME = /Users/maidokaara/Library/Android/sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)
• All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 10.0)
• Xcode at /Applications/Xcode-beta.app/Contents/Developer
• Xcode 10.0, Build version 10L221o
• ios-deploy 1.9.2
• CocoaPods version 1.5.3

[✓] Android Studio (version 3.1)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 27.1.1
• Dart plugin version 173.4700
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)

[✓] IntelliJ IDEA Ultimate Edition (version 2018.2.1)
• IntelliJ at /Applications/IntelliJ IDEA.app
• Flutter plugin version 27.1.3
• Dart plugin version 182.3911.37

[✓] VS Code (version 1.25.1)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 2.16.0

[✓] Connected devices (1 available)
• Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

• No issues found!

Issue 2:
Internal: b/143100923

When a Stack contains MyWidget inside of a Positioned.

Stack(
overflow: Overflow.visible,
children: [
Positioned(
top: -50.0,
child: MyWidget(),
)],
);

Since overflow is Overflow.visible, MyWidget displays outside of the Stack. However, it doesn't accept gestures in the overflowed area. I have no idea if it's easy, difficult or downright impossible to make this change, but I can think of a gazillion use cases for that, which at the moment must be satisfied by using the much more complex Overlay, or some Stack tricks.

And in case it's impossible to make this change, then at least the documentation should clearly state that overflowed content can't receive gestures.

Also, see: https://stackoverflow.com/questions/51366761/in-flutter-how-can-a-positioned-widget-feel-taps-outside-of-its-parent-stack-ar