Comment 11 for bug 1860921

Actually, the problem is the lack of XRender support in the tightvncserver Xserver.

xfwm4 is capable of running without XRender, but for some reason the mask of the pixmap end up being empty and therefore the pixmaps are all transparent.

Disabling XRender *and* XShape would work. But most gtk apps are not rendering well either in that setup so I am not sure that would take us very far.