Qt/Qml: how to make widget (createWindowContainer) fill parent widget - c++

Question

I have this problem for months. I try to implement a webview on iOS with QtWebView by Qml and embed the webview into a widget by QWidget::createWindowContainer (following a online tutorial). But I just can't get this widget to fill the parent widget, nor change its size at all.
I have write the following minimal example for explaining the problem:
main.cpp
#include <QApplication>
#include <QQuickView>
#include <QBoxLayout>
#include <QWidget>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget *mainwindow = new QWidget();
mainwindow->setStyleSheet("background-color:red;"); // so that we can see the problem more clearly
QHBoxLayout *layout = new QHBoxLayout(mainwindow);
QQuickView *view = new QQuickView();
QWidget* container = QWidget::createWindowContainer(view, mainwindow);
// container->setMinimumSize(200, 200);
// container->setMaximumSize(200, 200); // the displaying size doesn't change with or without these lines
// container->setFocusPolicy(Qt::TabFocus);
view->setSource(QUrl("qrc:///webview.qml"));
layout->addWidget(container);
mainwindow->setLayout(layout);
mainwindow->show();
return a.exec();
}
webview.qml
import QtQuick 2.2
import QtWebView 1.0
Item {
visible: true
anchors.fill: parent
WebView {
anchors.fill: parent
url: "https://www.google.de"
}
}
When I run this mini program on iphonesimulator in QtCreator, the layout is like the following screenshots (sorry I can't embed the pictures yet):
We can see the container doesn't fill the top area. And even when the parent widget is much smaller (in my actual program), the container has always this same size. When I rotate the iphonesimulator, it looks then like this!.
Could anyone please help to solve this? It's annoying me for months. Thanks!