We found a solution to that bug. The Cocoa window itself isn't set to non resizeable. If you do this, everything works as expected. class MainWindow : public QMainWindow { public: explicit MainWindow(QWidget *parent = nullptr) : QMainWindow(parent) { //this will get rid of the size grip widget showing setFixedSize(500, 500); } void mousePressEvent(QMouseEvent *) override { std::cout<<"mouse pressed"<<std::endl; } }; int main(int argc, char *argv[]) { QApplication a(argc, argv); MainWindow w; NSWindow* window = [(NSView*)w.window()->winId() window]; //IMTPORTANT. Otherwise the lower right mouse events will not be passed on correctly by cocoa window.styleMask &= ~NSWindowStyleMaskResizable; w.show(); return a.exec(); }

Thank you @JKSH .

@JKSH Yes, I had to reinstalled QT on Visual C++. Works OK... THANKS!!!