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

[ChrisW67]

Compare what I wrote:

remove the style sheet from Dialog1.ui and modify the palette in Designer so that QPalette::Window is Red.

to what you did in your code:

QPalette pal = palette();
pal.setColor(QPalette::Background, Qt::red);

QPalette pal = palette();
pal.setColor(QPalette::Background, Qt::red);

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You can actually make that palette adjustment to QPalette::Window in Designer if you wish. The QPalette::Window comes from the docs linked earlier.

The other thing I did in your project was apply a layout to everything in your Dialog1. I do not think this will make a difference to the painting only the layout and usability.