[PySide] Compositing/translucent windows problem with X11

[test010203 at secmail.pro]

Hello,

I need to display a PNG with a transparent background with python QT. It
should be possible to see the desktop and other program windows behind
this.

I tried to set transparency flags WA_TranslucentBackground and
WA_NoSystemBackground but nothing displays when I execute the script (no
error, no window, nothing).
QtX11Extras.QX11Info.isCompositingManagerRunning() also returns false. If
I dont set transparency flags, the PNG displays but background is black
not transparent.

I use Ubuntu 18.10 with LXQT with Compton compositor (transparency works
for regular browser, text editor, etc) but also tested with LXDE and XFCE
but same problems.

More infos here:
https://stackoverflow.com/questions/54494019/pyside2-doesnt-detect-compositing-x11-and-compton-no-transparency

What can I do to fix this?

Thanks for reading.