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

Code:

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auto*w=new Dialog1();
w->show();

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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

Code:

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auto*w=new Dialog1();
w->setWindowFlags(Qt::FramelessWindowHint); 
w->show();

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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/1vpu...e5SZAX46Z/view

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