GTK4 defaults to Vulkan, and an X11-forwarded display has no device to make a
swapchain from; the plot is a GtkGLArea, so it then asks for a GL context that
is not there either. --renderer cairo is the software preset for that display:
it exports GSK_RENDERER=cairo and LIBGL_ALWAYS_SOFTWARE=1 in main() before
gtk_init()/adw_init(), because both are read when the renderer and the context
are created. --renderer gl is the faster choice when indirect GL works, and
--renderer vulkan makes the default explicit. Nothing is exported, and GTK's
own choice stands, when the option is absent.
The once-a-second header line is now behind UDPSCOPE_PROBE_HEADER, following
the [notrace] rule: its widths are still sampled for the exit dump, but the
line is detail behind that reading rather than output every run prints.
UDPSCOPE_PROBE_RENDERER prints the exported pair before GTK starts and the
GskRenderer GTK actually built.
README, the manual's overview and docs/claude record the two symptoms and the
cure.
Martino Ferrari
fcf268d249