Why Isn depth buffering working at all?
Be sure you’re requesting a depth buffered-visual. If you set the MESA_DEBUG environment variable it will warn you about trying to enable depth testing when you don’t have a depth buffer. Specifically, make sure glutInitDisplayMode is being called with GLUT_DEPTH or glXChooseVisual is being called with a non-zero value for GLX_DEPTH_SIZE. This discussion applies to stencil buffers, accumulation buffers and alpha channels too.
