QMainWindow or QDialog show() - before displaying, the window area is painted white

[VVS]

auto*w=new Dialog1();
w->show();
QMainWindow or QDialog show() - before displaying, the window area is painted white. You may not notice this on fast computers. How to avoid this ?

If I use

auto*w=new Dialog1();
w->setWindowFlags(Qt::FramelessWindowHint);
w->show();
then there is no white paint, but there is no window header


Windows 10. Desktop Qt 5.15.0 MinGW 64-bit

Link to project https://drive.google.com/file/d/1vpuon7lIByZzgjJ0nBg8qU1e5SZAX46Z/view

Video
https://drive.google.com/file/d/1-gs5UrD62f_JIUFYu4zZftSaxCAbV9zr/view?usp=sharing