#include <QApplication>
#include <QMainWindow>
#include <QPushButton>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow w;
    // w.setWindowFlag(Qt::FramelessWindowHint, true);
    /* Adding this line will warn.
     * warn:setGeometry: Unable to set geometry 500x500+0+0 (frame: 500x500+0+0) on QWidgetWindow/"QMainWindowClassWindow" */
    QPushButton btn("btn");
    QObject::connect(&btn, &QPushButton::clicked, [&w](){
        w.resize(500, 500);
    });
    w.setCentralWidget(&btn);
    w.showMaximized();
    return a.exec();
}