Incorrect rendering of frameless window when used in conjunction with Windows Api

Helge1980

Hello!
I'm trying to draw a frameless window in older Windows versions (WinXp and Win7 with Qt5.2.1 MSVC 2010) with support for the system menu and system resizing, for this I use this approach:

#include <QApplication>
#include <QMainWindow>
#include <Windows.h>

class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget *parent = nullptr) : QMainWindow(parent)
    {
//        setWindowFlags(windowFlags() | Qt::FramelessWindowHint);
        setGeometry(600,300,600,400);
        HWND hwnd = (HWND)winId();
        LONG style = ::GetWindowLong(hwnd, GWL_STYLE) | WS_OVERLAPPEDWINDOW;
        ::SetWindowLong(hwnd, GWL_STYLE, style & ~WS_CAPTION);
        SetWindowPos(hwnd, 0, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_NOZORDER | SWP_NOACTIVATE | SWP_FRAMECHANGED);
    };
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWindow w;
    w.show();
    return app.exec();
}

But as a result, the window content displayed does not fill the entire client area:

If, for example, you minimize and then restore the window, the content size is displayed correctly.
If in my example I uncomment the line:

setWindowFlags(windowFlags() | Qt::FramelessWindowHint);

then the content, on the contrary, goes down more than necessary.

Thanks!