With nouveau driver on old Nvidia card, 6.0.5. on Manjaro,
I have experimented with this github install and found it working for window contents and decoration (each window or window class has to have manually added opacity rules, for active/inactive windows, in %).
It will show up in Desktop Effects as Force Blur.
-Untick the ~“add opacity” (or similar) in the first tab of its settings.
-In the other tabs select, apply to all except (above classes)
-Untick blur panel, menu, ~“fix something”, and all things except window (decoration).
-Temporarily disable the default install Desktop Effects: Translucency, Blur, Background Contrast, Slide Back, Magic Lamp.
- reboot once or twice even with logins
-then see if its working with a window that you gave an opacity rule.
Now you add the other desired effects that you disabled back and see it continuously working, after a logout/-in.
I’m not on my laptop right now but can make screen shots [of the above vaguely decribed] settings if desired!