How to make child widgets' corners rounded to match the outlines of the main GTK window?

[Helg1980 (Helg)]

Hello!
I’m trying to make a frameless GTK window with a Qt widget embedded inside it, but the widget ends up not matching the outline of the main GTK window. Is there any way to fix this?

Here’s a complete example of the code I’m using:

GtkMainWindow.pro

QT += core gui widgets
CONFIG += c++11
CONFIG += app_bundle
TEMPLATE = app
TARGET = gtkmainwindow

CONFIG  += link_pkgconfig
PKGCONFIG += gtk+-3.0

SOURCES += main.cpp gtkmainwindow.cpp
HEADERS += gtkmainwindow.h

gtkmainwindow.h

#ifndef GTKMAINWINDOW_H
#define GTKMAINWINDOW_H

#include <QWidget>
    
class GtkMainWindow
{
public:
    GtkMainWindow();
    ~GtkMainWindow();
    void setCentralWidget(QWidget *w);
    void show();

private:
    class GtkMainWindowPrivate;
    GtkMainWindowPrivate *pimpl;
};

#endif // GTKMAINWINDOW_H

gtkmainwindow.cpp

#include <gtk/gtk.h>
#include "gtkmainwindow.h"
#include <QVBoxLayout>
#include <gtk/gtkx.h>

gboolean on_configure_event(GtkWidget *wgt, GdkEvent *ev, gpointer data)
{
    if (QWidget *w = (QWidget*)data) {
        gint x = 0, y = 0;
        gtk_window_get_size(GTK_WINDOW(wgt), &x, &y);
        w->resize(x, y);
    }
    return FALSE;
}
   
class GtkMainWindow::GtkMainWindowPrivate
{
public:
    GtkMainWindowPrivate() {}
    ~GtkMainWindowPrivate() {}
    GtkWidget *window = nullptr;
    QWidget *w = nullptr;
};

GtkMainWindow::GtkMainWindow() :
    pimpl(new GtkMainWindowPrivate)
{
    gtk_init(NULL, NULL);
    pimpl->window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(pimpl->window), "GtkMainWindow");
    gtk_window_set_default_size(GTK_WINDOW(pimpl->window), 600, 400);
    gtk_window_set_position(GTK_WINDOW(pimpl->window), GtkWindowPosition::GTK_WIN_POS_CENTER);
    g_signal_connect(G_OBJECT(pimpl->window), "destroy", G_CALLBACK(gtk_main_quit), NULL);
    GtkCssProvider *provider = gtk_css_provider_new();
    gtk_css_provider_load_from_data(provider, "decoration {border: 0px solid #ff0000; border-radius: 8px 8px 0px 0px;}", -1, NULL);
    gtk_style_context_add_provider_for_screen(gdk_screen_get_default(), GTK_STYLE_PROVIDER(provider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);

    GtkWidget *header = gtk_header_bar_new();
    gtk_window_set_titlebar(GTK_WINDOW(pimpl->window), header);
    gtk_widget_destroy(header); // remove header
    
    GtkWidget *socket = gtk_socket_new();
    gtk_widget_show(socket);
    gtk_container_add(GTK_CONTAINER(pimpl->window), socket);

    pimpl->w = new QWidget(nullptr, Qt::FramelessWindowHint | Qt::X11BypassWindowManagerHint);
    g_signal_connect(G_OBJECT(pimpl->window), "configure-event", G_CALLBACK(on_configure_event), pimpl->w);
    QVBoxLayout *lut = new QVBoxLayout;
    lut->setContentsMargins(0,0,0,0);
    pimpl->w->setLayout(lut);
    pimpl->w->createWinId();
    gtk_socket_add_id(GTK_SOCKET(socket), (Window)pimpl->w->winId());
}

GtkMainWindow::~GtkMainWindow()
{
    delete pimpl;
}

void GtkMainWindow::setCentralWidget(QWidget *w)
{
    pimpl->w->layout()->addWidget(w);
}

void GtkMainWindow::show()
{
    gtk_widget_show_all(pimpl->window);
    pimpl->w->show();
}

main.cpp

#include <QApplication>
#include <QWidget>
#include <QGridLayout>
#include <QPushButton>
#include <QSpacerItem>
#include "gtkmainwindow.h"

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    GtkMainWindow wnd;
    QWidget *w = new QWidget;
    w->setStyleSheet("QWidget{background: #555555;} QPushButton{min-height: 24px; background: #888888; border: 0px solid #00ffff;}");
    QGridLayout *gl = new QGridLayout;
    w->setLayout(gl);
    gl->setContentsMargins(0,0,0,0);
    QPushButton *btn = new QPushButton("Button");
    gl->addWidget(btn, 0, 0, 1, 1);
    gl->addItem(new QSpacerItem(5, 5, QSizePolicy::Expanding, QSizePolicy::Expanding), 1, 0, 1, 1);
    gl->addWidget(new QPushButton("Button"), 2, 0, 1, 1);
    wnd.setCentralWidget(w);
    wnd.show();
    return app.exec();
}

There was an idea to use gdk_window_shape_combine_region() but it is not clear how exactly to do this and whether it would help.