GH-hosted ubuntu-latest cannot run window-bound GL tests:
- bare `xvfb-run cargo test` fails with `GLXBadWindow`: Xvfb's GLX
visuals lack `GLX_WINDOW_BIT`, so `glXCreateWindow` returns BadWindow.
- `xvfb-run -s "+extension GLX +render -noreset"` + `LIBGL_ALWAYS_SOFTWARE=1
GALLIUM_DRIVER=llvmpipe MESA_GL_VERSION_OVERRIDE=4.5` produced the same
GLXBadWindow error (run 25358253410): xvfb's GLX implementation does
not support `GLX_WINDOW_BIT` regardless of the software-rasterizer.
This is a known constraint across the Rust gfx ecosystem — bevy,
rust-skia and iced CI all skip window-bound GL tests on hosted Linux
runners and verify only `cargo build / test / clippy` link-time
correctness. The dep-stack probe's link half (skia-safe + glutin +
glow + winit) is already proven by the Linux `cargo build / test
/ clippy --all-targets` steps that pass before this gate.
Mirror the existing `WINDOWS_GPU_DEFERRED_NO_RUNNER` deferral pattern
(spec §8.2):
- probe test body early-returns with `LINUX_GPU_DEFERRED_NO_RUNNER`
when the env var is set; CI step exports it.
- locally on a real Linux desktop the env var is unset, so the full
cross-API state + readback verifications still run.
macOS retains the full window+GL path (CI + local), which alone
covers spec §7.2(2) "cross-API GL state visibility" and §6.2(c)
"full readback chain" — the only verifications that exercise live
GPU semantics. Windows + Linux on hosted runners verify the
toolchain links and the probe code compiles, which is what the
spec requires for those targets.
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