Window frame is visible (except on wayland), but transparent. It does react to closing the window, but it never draws anything as long as there is timer going. processEvents() call inside the timer callback does not help either.

Can be reproduced with this:

#include <QApplication>
#include <QTimer>
#include <QFileDialog>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QTimer t;
    t.start(0);
    QFileDialog::getOpenFileName();
}