diff --git a/mwp/lib/main.dart b/mwp/lib/main.dart
index 0fa454e..b4eab19 100644
--- a/mwp/lib/main.dart
+++ b/mwp/lib/main.dart
@@ -80,94 +80,3 @@ class _MyTooltipState extends State<MyTooltip> {
     );
   }
 }
-
-class Window extends StatefulWidget {
-  const Window({super.key, this.controller, required this.child});
-
-  final Widget child;
-  final WindowController? controller;
-
-  @override
-  State<Window> createState() => _WindowState();
-}
-
-class _WindowState extends State<Window> {
-  FlutterWindow? _window;
-  bool _windowNeedsConfigurationUpdate = false;
-
-  @override
-  void initState() {
-    super.initState();
-    widget.controller?.addListener(_handleWindowConfigurationChange);
-    _createWindow();
-  }
-
-  // TODO: didUpdateWidget (changing controller).
-
-  void _handleWindowConfigurationChange() {
-    if (_window == null) {
-      _windowNeedsConfigurationUpdate = true;
-    } else {
-      _window!.configuration = widget.controller!.toConfiguration();
-    }
-  }
-
-  Future<void> _createWindow() async {
-    final FlutterWindow window = await WindowingAPI.createWindow(
-      widget.controller?.toConfiguration() ?? WindowConfiguration(),
-    );
-    if (!mounted) {
-      window.dispose();
-      return;
-    }
-    setState(() {
-      _window = window;
-    });
-    if (_windowNeedsConfigurationUpdate) {
-      _windowNeedsConfigurationUpdate = false;
-      _window!.configuration = widget.controller!.toConfiguration();
-    }
-
-    // TODO: Connect the window with the window controller.
-  }
-
-  @override
-  void dispose() {
-    super.dispose();
-    _window?.dispose();
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    if (_window == null) {
-      // TODO: This doesn't work since there is no render tree to attach this to.
-      return const SizedBox();
-    }
-    return View(
-      view: _window!.view,
-      child: widget.child,
-    );
-  }
-}
-
-abstract class WindowController implements Listenable {
-  WindowConfiguration toConfiguration();
-}
-
-class WindowConfiguration {
-
-}
-
-// Lower level API.
-
-class WindowingAPI {
-  static Future<FlutterWindow> createWindow(WindowConfiguration config) async {
-
-  }
-}
-
-abstract class FlutterWindow {
-  FlutterView get view; // class could also extend FlutterView
-  WindowConfiguration configuration;
-  void dispose();
-}
diff --git a/mwp/lib/window.dart b/mwp/lib/window.dart
new file mode 100644
index 0000000..6f5733a
--- /dev/null
+++ b/mwp/lib/window.dart
@@ -0,0 +1,98 @@
+import 'package:flutter/material.dart';
+
+class Window extends StatefulWidget {
+  const Window({super.key, this.controller, required this.child});
+
+  final Widget child;
+  final WindowController? controller;
+
+  @override
+  State<Window> createState() => _WindowState();
+}
+
+class _WindowState extends State<Window> {
+  FlutterWindow? _window;
+  bool _pendingConfigurationUpdate = false;
+
+  @override
+  void initState() {
+    super.initState();
+    widget.controller?._attach(this);
+    _createWindow();
+  }
+
+  // TODO: didUpdateWidget (changing controller).
+
+  void _handleWindowConfigurationChange() {
+    if (_window == null) {
+      _windowNeedsConfigurationUpdate = true;
+    } else {
+      _window!.configuration = widget.controller!.toConfiguration();
+    }
+  }
+
+  Future<void> _createWindow() async {
+    final FlutterWindow window = await WindowingAPI.createWindow(
+      widget.controller?._configuration ?? WindowConfiguration(),
+    );
+    if (!mounted) {
+      window.dispose();
+      return;
+    }
+    setState(() {
+      _window = window;
+    });
+    if (_windowNeedsConfigurationUpdate) {
+      _windowNeedsConfigurationUpdate = false;
+      _window!.configuration = widget.controller!.toConfiguration();
+    }
+
+    // TODO: Connect the window with the window controller.
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    _window?.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (_window == null) {
+      // TODO: This doesn't work since there is no render tree to attach this to.
+      return const SizedBox();
+    }
+    return View(
+      view: _window!.view,
+      child: widget.child,
+    );
+  }
+}
+
+abstract class WindowController implements Listenable {
+  WindowController({
+
+});
+
+WindowConfiguration _configuration;
+
+void requestUpdate
+}
+
+class WindowConfiguration {
+
+}
+
+// Lower level API.
+
+class WindowingAPI {
+  static Future<FlutterWindow> createWindow(WindowConfiguration config) async {
+
+  }
+}
+
+abstract class FlutterWindow {
+  FlutterView get view; // class could also extend FlutterView
+  WindowConfiguration configuration;
+  void dispose();
+}
diff --git a/mwp/lib/window2.dart b/mwp/lib/window2.dart
new file mode 100644
index 0000000..5f53146
--- /dev/null
+++ b/mwp/lib/window2.dart
@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+
+class Window extends StatefulWidget {
+  const Window({super.key, this.controller, required this.child});
+
+  final WindowController? controller;
+  final Widget child;
+
+  @override
+  State<Window> createState() => _WindowState();
+}
+
+class _WindowState extends State<Window> {
+  @override
+  Widget build(BuildContext context) {
+    return const Placeholder();
+  }
+}
+
+class WindowController extends ChangeNotifier {
+  WindowController({
+    BoxConstraints constraints = const BoxConstraints.tightFor(width: 400, height: 800),
+  }) : _requestedConfiguration = _WindowConfigurationRequest(constraints: constraints);
+
+  _WindowConfigurationRequest _requestedConfiguration;
+  _WindowConfiguration? _configuration;
+
+  bool get isAttached => _configuration != null;
+
+  Size get size => _configuration!.size;
+}
+
+@immutable
+class _WindowConfigurationRequest {
+  const _WindowConfigurationRequest({
+    this.constraints,
+  });
+
+  final BoxConstraints? constraints;
+}
+
+@immutable
+class _WindowConfiguration {
+  const _WindowConfiguration({
+    required this.size,
+  });
+
+  final Size size;
+}
diff --git a/mwp/test/widget_test.dart b/mwp/test/widget_test.dart
index a20a2a0..4206eba 100644
--- a/mwp/test/widget_test.dart
+++ b/mwp/test/widget_test.dart
@@ -1,7 +1,10 @@
+import 'package:flutter/cupertino.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('Counter increments smoke test', (WidgetTester tester) async {
     expect(1, 1);
+    tester.pumpWidget(const Placeholder(), null, EnginePhase.build);
+    tester.pumpWidget();
   });
 }