do not compute experimental-webgl on OffscreenCanvas

src/backend/web-gl/kernel.js

@@ -62,7 +62,8 @@ class WebGLKernel extends GLKernel {
       testCanvas = new OffscreenCanvas(0, 0);
     }
     if (!testCanvas) return;
-    testContext = testCanvas.getContext('webgl') || testCanvas.getContext('experimental-webgl');
+    testContext = testCanvas.getContext('webgl');
+    if (!testContext && !(testCanvas instanceof OffscreenCanvas)) testContext = testCanvas.getContext('experimental-webgl');
     if (!testContext || !testContext.getExtension) return;
     testExtensions = {
       OES_texture_float: testContext.getExtension('OES_texture_float'),

test/issues/778-webgl-kernel-experimental.js

@@ -0,0 +1,29 @@
+const { assert, test, module: describe } = require('qunit');
+const { GPU } = require('../../src');
+
+const old = {};
+describe('issue #778 - WebGL kernel feature checks may throw an error', {
+    before: () => {
+        old.document = global.document;
+        old.OffscreenCanvas = global.OffscreenCanvas;
+
+        global.document = undefined;
+        // Mocking OffscreenCanvas
+        global.OffscreenCanvas = class OffscreenCanvas {
+            constructor() {}
+
+            getContext(context) {
+                if (context === "webgl") return;
+                if (context === 'experimental-webgl') throw new TypeError("Failed to execute 'getContext' on 'OffscreenCanvas': The provided value 'experimental-webgl' is not a valid enum value of type OffscreenRenderingContextType.")
+            }
+        }
+    },
+    after: () => {
+        global.document = old.document;
+        global.OffscreenCanvas = old.OffscreenCanvas;
+    }
+});
+
+test('Check that WebGL is not supported', () => {
+    assert.notOk(GPU.isWebGLSupported);
+});