Re: nvidia staff PLEASE definitively answer triple head/dual card questions
This has to do with the way Xinerama information is reported to X clients. In TwinView, the NVIDIA driver provides its own "fake" Xinerama extension to tell the clients where the screens are. When you add a third screen and turn on the X server's real Xinerama extension, that takes over. To it, the TwinView screen shows up as one big desktop, and it only advertises two screens to the clients. This is a fundamental limitation in the way Xinerama works right now. There are plans to improve support for TwinView/mergedfb plus Xinerama configurations in version 1.2 of the RandR extension, but until that is ready, we're stuck with this limitation.