NV-driver Linux cannot-get-Z-buffer for legacy-OpenGL

Graphics / Linux Linux
1 
Gtk3 + GLX cannot get Z-buffer - only 2D;
The driver nouveau works fine.
See minimal-code-program 
https://github.com/gcad3d/gcad-GL/blob/master/tests_gl_legacy/gtk3_glx   
 (build with make -f gtk3_glx.mak  - using gtk3_glx.c)   

Tested on:
- NV-Driver 390.144
- and NV-Driver 460.91.03
- and NV-Driver 470.86
firmware:       nvidia/470.86/gsp.bin
alias:          char-major-195-*
version:        470.86

- on Ubuntu 20.04.3 LTS (Focal Fossa) 64-bit and also Ubuntu 18;
  (Kernel Linux 5.11.0-43-generic x86_64)
- with NVIDIA GeForce GTX 1050 Ti/PCIe/SSE2

Maybe the problem is in Gtk3; ??   
Franz
2

Maybe also ask in the OpenGL forum:
https://forums.developer.nvidia.com/c/gaming-and-visualization-technologies/apis/opengl/199