Re: Computer freezes when mplayer window off screen + composite problems
The composite extension is still under heavy development. It doesn't work together with Xv and GLX. There have been discussions on the xorg lists to what needs to be done to address these problems. They are not really nvidia's fault. Further nvidia marked composite as an experimental feature for reasons, they knew about these problems. (read the driver doc, it talks about Xv/GLX issues)
Just don't use composite till the problems have been fixed in xorg and then I'm sure nvidia will update their drivers to fix the problems.