The main window is not transparent under Linux with the Xfce desktop environment.
Instead, the window has a black background and the opacity setting appears to set the transparency of the chosen Background Color in the preferences over this black background. For example, the above has a White Background Color set at about 50% opacity.
This may provide some direction in solving the issue:
http://stackoverflow.com/questions/18316710/frameless-and-transparent-window-qt5
However, if any of these additional flags are set, regression testing should be done on at least the Linux desktop environments where the transparency already works, to make sure it doesn't fix one DE and break another.
The main window is not transparent under Linux with the Xfce desktop environment.
Instead, the window has a black background and the opacity setting appears to set the transparency of the chosen Background Color in the preferences over this black background. For example, the above has a White Background Color set at about 50% opacity.
This may provide some direction in solving the issue:
http://stackoverflow.com/questions/18316710/frameless-and-transparent-window-qt5
However, if any of these additional flags are set, regression testing should be done on at least the Linux desktop environments where the transparency already works, to make sure it doesn't fix one DE and break another.