I'm also having this issue on Ubuntu 18.04 amd64, compiz 1:0.9.13.1+18.04.20180302-0u, proprietary nVidia drivers 415.27-0ubuntu0~gpu18.04.2 on an old GeForce GTX 750.
This Gimp bug seems related: https://bugs.launchpad.net/ubuntu/+source/gimp/+bug/1371928 (and I'm having it too). I also have transparent windows on emulated wine windows.
I'm also having this issue on Ubuntu 18.04 amd64, compiz 1:0.9.13. 1+18.04. 20180302- 0u, proprietary nVidia drivers 415.27- 0ubuntu0~ gpu18.04. 2 on an old GeForce GTX 750.
This Gimp bug seems related: https:/ /bugs.launchpad .net/ubuntu/ +source/ gimp/+bug/ 1371928 (and I'm having it too). I also have transparent windows on emulated wine windows.