Raspberry Pi Transparent QWidget Problem

[qtross]

Hello to all,
I am trying to create a transparent widget. I searched it and found a solution which works without problem on linux.

setWindowFlags(Qt::Widget | Qt::FramelessWindowHint);
setParent(0); // Create TopLevel-Widget
setAttribute(Qt::WA_NoSystemBackground, true);
setAttribute(Qt::WA_TranslucentBackground, true);

QTCreator version on linux is 5.11.3.
However, when i sent the codes and try it on Raspberry pi 4 rasbian buster os, the transparent area stays black when i run the program. Qt Creator version on rasbian is same : 5.11.3

Then , I tried adding this;

setAttribute(Qt::WA_PaintOnScreen); 

It works but it makes every object transparent and gives error while running program as;

Paint device returned engine == 0, type: 1

May the X server on rasbian cause the problem?
Thank you for any advice.

[VaL Doroshchuk]

Not sure, but maybe need to set alpha buffer size?
QSurfaceFormat format;
format.setAlphaBufferSize(8);
window.setFormat(format);