#include <QtWidgets>
#include <QMainWindow>
#include <QtGui>
#include <Windows.h>

class TChild : public QWidget
{
public:
	TChild(QWidget* parent, QWidget* other)
		:
		QWidget(parent, Qt::Dialog),
		m_other(other)
		{
		}
		void mouseReleaseEvent(QMouseEvent*)
		{
			setParent(m_other, windowFlags());
			// SetWindowLongPtr((HWND)winId(), GWL_HWNDPARENT, m_other ? (LONG_PTR)m_other->winId() : 0);
			show();
		}
	QWidget* m_other;
};
 
int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	QWidget* window = new QWidget(0, Qt::Window);
	window->setWindowTitle("Parent 1");
	window->resize(400,400);
	window->move(100,100);
	window->show();

	QWidget* window2 = new QWidget(0, Qt::Window);
	window2->setWindowTitle("Parent 2");
	window2->resize(400,400);
	window2->move(200,200);
	window2->show();

	TChild* window3 = new TChild(window2, window);
	window3->setWindowTitle("Child");
	window3->resize(200,200);
	window3->move(300,300);
	window3->show();

	return app.exec();
}