After closing windows, I see messages like 2013-10-02 16:28:52.990 firefox[6109:9507] invalid context printed to the console. They come from inside [NSOpenGLContext currentContext] when the most recent current context has been released while it was still current.

This fixes it. The [mContext release] call in ~GLContextCGL() does not actually deallocate the context because the ChildView holds another reference to it, but calling clearCurrentContext in -[ChildView dealloc] (which has the context-deallocating [mGLContext release] call) is no use because currentContext-ness is per thread, and the context is actually current on the Compositor thread and not on the main thread where the -[ChildView dealloc] happens.