flutter: Only DropArea creates the stack now

It was a bad default. Other widgets can fill parent.

src/flutter/dart/lib/DropArea_flutter.dart

@@ -21,6 +21,10 @@ class DropArea_flutter extends View_flutter {
       : super(controller, type, parent, windowFlags: windowFlags) {
     print("DropArea_flutter CTOR");
   }
+
+  Widget createFlutterWidget() {
+    return DropAreaWidget(kddwView, this, key: widgetKey);
+  }
 }
 
 class DropAreaWidget extends PositionedWidget {
@@ -42,6 +46,10 @@ class DockWidgetPositionedWidgetState extends PositionedWidgetState {
 
   @override
   Widget buildContents() {
-    return super.buildContents();
+    return Container(
+        color: kddwView.m_color,
+        child: Stack(
+          children: kddwView.childWidgets,
+        ));
   }
 }

src/flutter/dart/lib/PositionedWidget.dart

@@ -100,11 +100,7 @@ class PositionedWidgetState extends State<PositionedWidget>
 
   /// This is factored-out from build() so derived classes can return something else
   Widget buildContents() {
-    return Container(
-        color: kddwView.m_color,
-        child: Stack(
-          children: kddwView.childWidgets,
-        ));
+    return Container(color: kddwView.m_color);
   }
 
   @override