https://bugs.winehq.org/show_bug.cgi?id=39314
--- Comment #11 from Tobias (:Tobbi) Markus tobbi.bugs@googlemail.com --- (In reply to Ken Thomases from comment #10)
Try setting this registry value:
[HKCU\Software\Wine\Mac Driver] "OpenGLSurfaceMode"="transparent"
This didn't help, unfortunately. I did change the setting to "behind" just to check whether it was having any effect at all, which it did. It just didn't fix the problem.