How to avoid QWidget show the default background before paintEvent be called

[wgong]

Envirment

Win10 + Qt15.4.2 + QWidget Application

Problem

I Create a empty QWidget Application,and put some child widget on it, the rewrite it's paintEvent to fill window to blue.
But when run the program,I found:

int main(int argc, char* argv[]){
    QApplication a(argc, argv);
    MyWidget w;
    w.show();    //After this, w.paintEvent is not called, the widget is shown using default style,such as a white background,but the child widget isn't shown.
    a.exec();    //After this, w.paintEvent is called, the background is be painted to blue. and child widget is shown.
}

That is when start, white background without child widget is shown first, soon, it changed to blue background with all child widget. A serious flash。

Question

How can we avoid this flash? Is it possible to show the widget with all child is shown and paintEvent had been called?

[wgong]

And I found the official example of Qt: Style Example is the same。

Example

After show() and before exex():

After exec():

[Christian Ehrlicher]

@wgong You can't. Show sends an event to the window manager and it is showing the decoration. It's just a debugging issue.

[JoeCFD]

@wgong Qt15.4.2 does not exist. Is it Qt5.14.2? If yes, upgrade your Qt to at least 5.15.2 first. Is your screen a touch screen?

[wgong]

@JoeCFD , 5.14.2 is right. I am sorry, it's a clerical error. I will try other version later,and hope somethind is different.thanks.

[wgong]

@Christian-Ehrlicher
Thanks for reply. But it not only a debug issue. I test the release mode, the default background appears first, then final background.obvious flash,same as the debug mode.

[wgong]

@JoeCFD
oh, I am not a touch screen. It is a pc with a monitor.