Generally VSync in full screen should work, one thing that's some kind awkward is that GLB starts glBegin right after flipping buffers.. Problem may be dependent from GPU/OS/drivers combination, as mentioned try forcing VSync in gpu drivers control panel. On older Vega iGPU I don't see issues (full screen, 60Hz display, fps set to 60-120 always limited by vsync to 60 without screen tearing).
You can also check stability of rendering frames with some software like MSI Afterburner or new Intel PresentMon.
You can also check stability of rendering frames with some software like MSI Afterburner or new Intel PresentMon.