diff --git a/crates/gpu/src/context.rs b/crates/gpu/src/context.rs
index 7166302d..93283e78 100644
--- a/crates/gpu/src/context.rs
+++ b/crates/gpu/src/context.rs
@@ -335,6 +335,11 @@ impl GpuContext {
         if !adapter.get_downlevel_capabilities().flags.contains(wgpu::DownlevelFlags::COMPUTE_SHADERS) {
             return Err(format!("{} ({:?}): no compute shaders", info.name, info.backend));
         }
+        // OpenGL (Window Graphics ▸ OpenGL for drivers that crash otherwise, #243) translates the
+        // kernels for tens of seconds before one fails validation: CPU compositing at once.
+        if info.backend == wgpu::Backend::Gl {
+            return Err(format!("{} (Gl): the GPU compositor needs DirectX 12, Vulkan, Metal or WebGPU", info.name));
+        }
         // egui-wgpu asks for WebGL2's limits on GL (no storage buffers at all): building the
         // pipelines there would be a validation error, which release builds only log.
         let l = device.limits();
diff --git a/crates/gpu/src/tests_init.rs b/crates/gpu/src/tests_init.rs
index 38c278c2..9c579ae6 100644
--- a/crates/gpu/src/tests_init.rs
+++ b/crates/gpu/src/tests_init.rs
@@ -136,6 +136,19 @@ fn initialization_unwind_drains_all_scopes_before_the_next_operation() {
 
 /// Acceptance for the captured RTX 5090 / FXC failure. Future compiler/shader fixes can
 /// legitimately make FXC succeed; this diagnostic is deliberately outside portable CI.
+/// On OpenGL the compositor declines at once instead of translating its kernels for tens of
+/// seconds and then failing (#243). Skips where there is no OpenGL adapter.
+#[test]
+fn opengl_adapters_get_cpu_compositing_at_once() {
+    let mut desc = wgpu::InstanceDescriptor::new_without_display_handle();
+    desc.backends = wgpu::Backends::GL;
+    let Some((adapter, device, queue)) = native_device(desc, false) else { return };
+    let t = std::time::Instant::now();
+    let error = crate::Gpu::new(&adapter, device, queue).err().expect("no GPU compositor on OpenGL");
+    assert!(error.contains("(Gl)"), "{error}");
+    assert!(t.elapsed() < std::time::Duration::from_secs(1), "{:?}", t.elapsed());
+}
+
 #[test]
 #[cfg(target_os = "windows")]
 #[ignore = "requires the reproduced Windows DX12/FXC pointwise compiler failure; no adapter skips"]