Constant NVIDIA_GLX 

const NVIDIA_GLX: &str = "NV-GLX";

The extension the NVIDIA driver’s GLX registers, and the one libnvidia-fbc resolves on its own X connection before it will do anything: NvFBC captures the screen through a GLX context only that driver provides. An X server without it — an Xvfb, a Mesa-driven Xorg — has nothing for the library to attach to, and the lookup there is not merely futile: it can block inside the library and never return, which costs the capture thread and then the display’s stop path.