My 2 cents: I'd prefer to not flag an error: this is low level
enough that
I don't expect anyone using it to be surprised when it doesn't work on X11
frames, and I don't see any benefit in forcing people to conditionalize
calls to it to avoid the error message. That it only works on native
windows frames could be mentioned in the docstring.
Fair enough. I've posted a patch to xemacs-patches. Fingers crossed
that it gets installed :)