Geometry of native widgets breaks when I put them into a non-native QTabWidget

Qt 6.9.2, Windows.

I put a native widget (made native with winId()) into a QTabWidget, which is a child of some root QWidget.
It works OK, but if I add more widgets, their geometry breaks.
The next widgets have regions on the right and bottom sides that are rendered with random garbage.
I have this problem on Qt 6.9.2, but the same code works fine on Qt 5.12.3.

Here's a minimal reproduction:

Code:

---

#include <QApplication>
#include <QPushButton>
#include <QTabWidget>
#include <QStackedWidget>
#include <QVBoxLayout>
 
int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
 
    auto button = new QPushButton("buton");
    button->resize(200, 200);
    button->show();
 
    auto window = new QWidget{};
    auto main_layout = new QVBoxLayout(window);
    window->resize(500,500);
    window->show();
 
    QObject::connect(button, &QPushButton::clicked, [&]() {
        auto widget = new QWidget();
        widget->setStyleSheet("QWidget { background-color: #119911; }");
        widget->winId(); // make it native
 
        auto tab_widget = new QTabWidget(window);
        // tab_widget->winId(); // fixes the problem
        tab_widget->addTab(widget, "mytab");
 
        main_layout->addWidget(tab_widget);
    });
 
    return QApplication::exec();
}

---

Attachment 13872

I fixed the problem by making QTabWidget native first.
I could also use Qt::WA_DontCreateNativeAncestors, but I ran into other problems with scroll areas, so I can't use this attribute.

It looks like it has something to do with the window frame. The size of the garbage area suspiciously matches the height of the window frame.
If I use the Qt::FramelessWindowHint window flag for the widget, it fixes the problem in the code above. But it doesn’t help in my actual code, where the widget hierarchy is more complicated.

I inspected the native windows of widget and its parent with Spy++: the rectangles don't match, even though the widgets have the same geometry in GammaRay. So the native windows and Qt widgets got out of sync for some reason.

Attachment 13871