I use this code to change the transparency of a dockwiget.
I had no problem with qt5 until qt6.5.
Since a qt6.6  update , Qcolor no longer has any effect.

   // draw background
              QColor backgroundColor = palette.color( QPalette::Window );
               backgroundColor.setAlphaF( StyleConfigData::SidebarOpacity()/100.0 );
               painter.setBrush( backgroundColor );