How to change transparency of X11 window under Mac OS X (Xquartz)?

Question

XChangeProperty() with atom named _NET_WM_WINDOW_OPACITY returns BadRequest error code...

Answer 1

_NET_WM_WINDOW_OPACITY works by having the compositing manager apply opacity when it composites the window. You would need to be running a compositing manager that does this. (That said, even if you have no CM that's going to use the property, you should not get BadRequest; the BadRequest is likely due to a bug in how you're doing XChangeProperty, like an invalid window or atom or format)

Another thing you can try to do is just create your window with an ARGB visual and then paint alpha channel into the window itself. How to do this depends on which toolkit you're using, etc. And it's only possible if your X server has an ARGB visual, I don't know if Mac does.