flutter: fix incorrect rendering due to not using keys

Flutter would reuse an arbitrary GroupWidget when rebuilding
the tree

src/flutter/dart/lib/widgets/DockWidget.dart

@@ -14,7 +14,7 @@ import 'package:KDDockWidgets/KDDockWidgets.dart';
 
 class DockWidget extends StatefulWidget {
   final DockItem dockItem;
-  DockWidget(this.dockItem, {Key? key}) : super(key: key) {}
+  DockWidget(this.dockItem) : super(key: ObjectKey(dockItem)) {}
 
   @override
   State<DockWidget> createState() {

src/flutter/dart/lib/widgets/FloatingWidget.dart

@@ -20,7 +20,7 @@ import 'package:flutter/material.dart';
 
 class FloatingWidget extends StatefulWidget {
   final FloatingItem floatingItem;
-  FloatingWidget(this.floatingItem, {Key? key}) : super(key: key);
+  FloatingWidget(this.floatingItem) : super(key: ObjectKey(floatingItem));
 
   @override
   State<FloatingWidget> createState() {

src/flutter/dart/lib/widgets/GroupWidget.dart

@@ -21,7 +21,7 @@ import 'package:signals_slots/signals_slots.dart';
 
 class GroupWidget extends PositionedWidget {
   final Group group;
-  GroupWidget(this.group, {Key? key}) : super(group, key: key);
+  GroupWidget(this.group) : super(group, key: ObjectKey(group));
 
   @override
   State<PositionedWidget> createState() {

src/flutter/dart/lib/widgets/SeparatorWidget.dart

@@ -17,7 +17,7 @@ import 'PositionedWidget.dart';
 
 class SeparatorWidget extends PositionedWidget {
   final Separator separator;
-  SeparatorWidget(this.separator, {Key? key}) : super(separator, key: key);
+  SeparatorWidget(this.separator) : super(separator, key: ObjectKey(separator));
 
   @override
   State<PositionedWidget> createState() {

src/flutter/dart/lib/widgets/TabBarWidget.dart

@@ -19,7 +19,7 @@ import 'dart:ui';
 
 class TabBarWidget extends StatefulWidget {
   final Group group;
-  TabBarWidget(this.group, {Key? key}) : super(key: key);
+  TabBarWidget(this.group) : super(key: ObjectKey(group));
 
   @override
   State<TabBarWidget> createState() {

src/flutter/dart/lib/widgets/TitleBarWidget.dart

@@ -16,7 +16,7 @@ import 'package:flutter/material.dart';
 
 class TitleBarWidget extends StatefulWidget {
   final TitleBar titlebar;
-  TitleBarWidget(this.titlebar, {Key? key}) : super(key: key);
+  TitleBarWidget(this.titlebar) : super(key: ObjectKey(titlebar));
 
   @override
   State<TitleBarWidget> createState() {

tests/flutter/integration_test/ui_test.dart

@@ -95,4 +95,39 @@ void main() async {
     await tester.pump();
     await saveScreenShot(tester, prefix: "basic");
   });
+
+  testWidgets('TitleBar close button', (WidgetTester tester) async {
+    final dock1 = DockItem(uniqueName: "dw1", guestWidget: MyWidget());
+    final dock2 = DockItem(uniqueName: "dw2", guestWidget: MyWidget());
+
+    final dropArea = DropArea();
+    dropArea.addDockItem(dock1, Location.LocationOnTop);
+    dropArea.addDockItem(dock2, Location.LocationOnBottom);
+
+    dropArea.setLayoutSize(700, 700);
+    final dropAreaWidget = DropAreaWidget(dropArea);
+
+    // Build the widget tree
+    await tester.pumpWidget(MyApp(
+      child: dropAreaWidget,
+    ));
+
+    await tester.pump();
+    await saveScreenShot(tester, prefix: "titlebar-close-button");
+
+    expect(dropArea.groups.length, 2);
+    var topGroup = dropArea.groups.first;
+    var bottomGroup = dropArea.groups.last;
+
+    topGroup.titlebar.onCloseClicked();
+    expect(dropArea.containsGroup(topGroup), false);
+    expect(dropArea.containsGroup(bottomGroup), true);
+    expect(dropArea.groups.length, 1);
+    expect(dropArea.groups.first, bottomGroup);
+    expect(bottomGroup.numDockWidgets(), 1);
+    expect(bottomGroup.dockWidgetAt(0), dock2);
+
+    await tester.pump();
+    await saveScreenShot(tester, prefix: "titlebar-close-button-2");
+  });
 }